Background

Current data on the routinely occurring peaks of deaths from suicide are conflicting [1,2]. However, for the past four decades in Finland, the seasonal pattern has been stronger the lower the suicide mortality has been [3]. There is a clear peak of suicide occurrence around May or June [4 - 7] and a preceding peak in suicide attempts around April [8]. Furthermore, another smaller peak of suicide occurrence exists around October [7,9]. These two mortality peaks, being similar and more robust the further away the country locates from the equator, have been explained by socio-demographic and socio-economic factors [10], but since this seasonal pattern has existed for decades [11], if not centuries [12], biological factors are likely.

Major depressive episodes are known to contribute to suicide substantially [13,14], and a history of mood disorders and psychiatric hospitalization associates clearly with the seasonal occurrence of suicides [15,16]. Desynchronization of physiological rhythms, e.g. desynchronization of the circadian rhythm of core body temperature with the sleep-wake cycle [17 - 19] and some clock gene variants [20,21], can be associated with mood disorders. Based on our earlier psychological autopsy studies of death from suicide [22] and the data from the nationwide suicide program in Finland [23], we hypothesized that the circadian misalignment among the depressed may increase during spring, and thereby predispose to suicidal behaviors [24].

Rest-activity cycles during the day [25] and sleep stages at night [26] are controlled by circadian clocks, but they are frequently disturbed among the depressed. Furthermore, the principal circadian clock entrains to the sun light [27 - 29], by tracking the daily changes in rise and set times of the sun and the variation in the length of the day [30 - 32]. Thus, the timing of light exposure is relevant to entrainment and influences the course of mood disorders [33,34]. Therefore, we hypothesized that it is the key to the suicide mortality peaks whether the light-dark transitions give the principal circadian pacemaker a signal to accelerate or decelerate, especially among the depressed. In addition, since sunshine and ambient temperature are potential time-givers, modulate the function of biological clocks [35], and associate with deaths from suicide [3,36], we aimed to test their effect, as well.

Methods

Statistics Finland http://www.stat.fi provided us with the daily data of 43,393 suicides, 33,993 of men and 9400 (22%) of women, committed in Finland during the 35-year period of 1969 to 2003 (Tables 1 and 2). Two phenomena, which affect the timing and the speed of the light-dark transitions regularly each year, were selected a priori as the potential factors that might challenge the biological clocks and produce circadian misalignment. First, we focused on the length of the photoperiod, because at high to temperate latitudes around spring and fall equinoxes the transitions between day and night are most rapid and the durations of twilight short, as a consequence of the rotation of the earth. Second, we focused on the constant mismatch between the sun time (hereafter ST) and the coordinated universal time (hereafter UCT), arising from the earth's tilt and elliptical orbit around the sun.

Table 1

Men's suicides in numbers during the study period

Oulu Helsinki Finland
Year Men S per
100
000 		Men S per
100
000 		Men S per
100
000
1969 42 447 11 25.92 272 321 110 40.39 2 230 217 850 38.11
1970 41 412 13 31.39 266 174 108 40.58 2 219 985 763 34.37
1971 42 798 11 25.70 271 393 117 43.11 2 234 037 781 34.96
1972 43 436 22 50.65 275 378 132 47.93 2 249 051 874 38.86
1973 44 127 27 61.19 277 205 109 39.32 2 262 142 849 37.53
1974 45 082 22 48.80 278 485 131 47.04 2 273 815 921 40.51
1975 45 815 22 48.02 278 628 128 45.40 2 282 115 924 40.49
1976 46 069 29 62.95 278 693 152 54.54 2 286 392 967 42.29
1977 46 444 22 47.37 277 978 154 55.40 2 295 668 962 41.91
1978 46 609 13 27.89 277 735 156 56.17 2 300 790 963 41.86
1979 46 533 18 38.68 278 569 133 47.74 2 306 784 935 40.53
1980 46 779 24 51.31 279 456 145 51.89 2 314 843 962 41.56
1981 47 343 21 44.36 280 580 151 53.82 2 327 473 904 38.84
1982 48 179 18 37.36 282 751 134 47.39 2 342 869 905 38.63
1983 48 331 25 51.73 284 565 130 45.68 2 357 172 938 39.79
1984 48 620 25 51.42 286 092 149 52.08 2 369 228 988 41.70
1985 49 065 23 46.88 287 858 113 39.26 2 377 780 964 40.54
1986 49 405 30 60.72 290 370 149 51.31 2 385 866 1023 42.88
1987 49 890 28 56.12 292 935 137 46.77 2 392 868 1068 44.63
1988 50 138 44 87.76 294 242 150 50.98 2 401 368 1112 46.31
1989 50 951 29 56.92 295 665 160 54.12 2 412 760 1121 46.46
1990 51 623 33 63.93 298 420 198 66.35 2 426 204 1199 49.42
1991 52 254 35 66.98 302 609 185 61.14 2 443 042 1193 48.83
1992 52 959 36 67.98 306 298 204 66.60 2 457 282 1160 47.21
1993 53 495 35 65.43 311 134 172 55.28 2 470 196 1112 45.02
1994 54 661 23 42.08 316 367 176 55.63 2 481 649 1080 43.52
1995 56 132 26 46.32 322 074 179 55.58 2 491 701 1081 43.38
1996 57 436 26 45.27 327 168 131 40.04 2 500 596 966 38.63
1997 58 482 36 61.56 332 113 158 47.57 2 509 098 1039 41.41
1998 59 606 26 43.62 337 297 121 35.87 2 516 075 965 38.35
1999 61 025 40 65.55 341 125 139 40.75 2 523 026 961 38.09
2000 62 800 28 44.59 344 520 143 41.51 2 529 341 879 34.75
2001 64 116 31 48.35 347 925 150 43.11 2 537 597 936 36.89
2002 64 995 22 33.85 349 121 139 39.81 2 544 916 825 32.42
2003 65 965 29 43.96 350 334 119 33.97 2 552 893 823 32.24

The yearly male population, number of suicides(S), and suicide mortality for men in Oulu, Helsinki, and Finland from 1969 to 2003.

Table 2

Women's suicides in numbers during the study period

Oulu Helsinki Finland
Year Women S per
100
000 		Women S per
100
000 		Women S per
100
000
1969 46 245 5 10.81 333 502 43 12.89 2 384 060 246 10.32
1970 45 656 4 8.76 325 034 58 17.84 2 378 351 220 9.25
1971 46 474 5 10.76 330 205 58 17.57 2 391 875 222 9.28
1972 47 633 5 10.50 333 507 57 17.09 2 404 350 239 9.94
1973 48 302 6 12.42 335 687 60 17.87 2 416 619 249 10.30
1974 49 272 11 22.33 337 470 47 13.93 2 428 572 255 10.50
1975 50 132 5 9.97 337 570 72 21.33 2 438 377 254 10.42
1976 50 410 12 23.81 336 980 66 19.59 2 444 444 253 10.35
1977 50 691 8 15.78 335 057 54 16.12 2 451 299 258 10.53
1978 50 964 11 21.58 334 547 53 15.84 2 457 298 237 9.65
1979 51 188 6 11.72 334 981 60 17.91 2 464 508 242 9.82
1980 51 582 4 7.76 335 630 63 18.77 2 472 935 264 10.68
1981 52 237 1 1.91 336 511 56 16.64 2 484 677 239 9.62
1982 53 059 10 18.85 338 116 59 17.45 2 498 846 267 10.69
1983 53 225 6 11.27 339 108 56 16.51 2 512 686 249 9.91
1984 53 443 3 5.61 340 162 54 15.88 2 524 520 253 10.02
1985 53 976 4 7.41 341 781 58 16.97 2 532 884 249 9.83
1986 54 349 12 22.08 343 576 53 15.43 2 539 778 287 11.30
1987 54 760 10 18.26 346 162 65 18.78 2 545 734 301 11.82
1988 55 125 9 16.33 346 880 62 17.87 2 552 991 296 11.59
1989 55 810 8 14.33 347 226 58 16.70 2 561 623 297 12.59
1990 56 294 8 14.21 348 913 90 25.79 2 572 274 324 11.60
1991 56 735 8 14.10 352 207 74 21.01 2 585 960 306 11.83
1992 57 391 6 10.46 354 429 77 21.73 2 597 700 297 11.43
1993 57 765 6 10.39 358 557 72 20.08 2 607 716 293 11.24
1994 58 781 12 20.42 363 774 77 21.17 2 617 105 307 11.73
1995 60 186 12 19.94 369 437 65 17.59 2 625 125 309 11.77
1996 61 447 8 13.02 373 663 69 18.47 2 631 724 282 10.72
1997 62 439 11 17.62 378 547 68 17.96 2 638 251 284 10.77
1998 63 454 11 17.34 382 880 57 14.89 2 643 571 268 10.14
1999 64 516 13 20.15 386 384 68 17.60 2 648 276 254 9.59
2000 66 149 11 16.63 389 425 66 16.95 2 651 774 292 11.01
2001 67 584 9 13.32 391 649 62 15.83 2 657 304 271 10.20
2002 68 297 11 16.11 392 485 48 12.23 2 661 379 275 10.33
2003 68 878 7 10.16 393 035 55 13.99 2 666 839 261 9.79

The yearly female population, number of suicides(S), and suicide mortality for women in Oulu, Helsinki, and Finland from 1969 to 2003.

The nominal calendar year was split into twelve periods according to these two phenomena; first into four astronomical seasons, which are determined by spring and fall equinoxes and summer and winter solstices (for the definition, see http://asa.nao.rl.ac.uk/), and second into eight periods, by the equation of time (for the definition, see http://www.nmm.ac.uk/explore/astronomy-and-time/time-facts/the-equation-of-time), as follows (see also Figure 1). From February 11th to May 14th (hereafter marked as X1) and from July 26th to November 3rd (X2) ST goes fast compared with UCT and in between those periods, that is, from May 15th to July 25th (Y1) and from November 4th to February 10th (Y2) it goes slow. Furthermore, another categorization was made based on the equation of time separating periods when ST is either ahead or behind the UCT. In other words, ST is constantly ahead of the UCT, from April 15th to June 13th (A1) and from September 1st to December 25th (A2), and constantly behind the UCT, from June 14th to August 31st (D1), and from December 26th to April 14th (D2). Hence, ST deviates from UCT constantly and is maximally behind at February 11th (approximately 14 minutes) and vice versa maximally ahead at November 3rd (approximately 16 minutes). The Almanac Office at the University of Helsinki http://almanakka.helsinki.fi/ both provided the dates for the astronomical seasons and calculated the dates for the periods (X1, Y1, X2, Y2) of the equation of time, as well as the dates for the periods (A1, D1, A2, D2) through the whole study period.

Figure 1 

Periods according to time of equation and astronomical seasons. During X1(February 11-May 04) and X2 (July 26-November 03) (marked with pink lines) sun time is accelerating, and during Y1 (May 15-July25) and Y2 (November 04-February 10) (marked with green lines) it is decelerating compared with the coordinated universal time (UCT). During A1 (April 15-June13) and A2 (September 01-December 25) sun time stays ahead and during D1 (June 14-August 31) and D2 (December 26-April 14) it stays behind the UCT. Astronomical seasons are separated with dotted vertical lines. During astronomical spring and summer daylight exceeds darkness, and vice versa during astronomical fall and winter darkness exceeds daylight in Finland. Y-axis on the left side presents the time difference (in minutes) that sun time deviates from the UCT.

To evaluate the effect of daily sunshine hours and temperature on suicide mortality, we focused on two cities on a similar longitude but with dissimilar photoperiod: first, Helsinki (60°9.7'N, 24°57.3'E), which is the capital of Finland in the south, and second, Oulu (65°1.0'N, 25°30.0'E), which is a central city of the northern part of the country, 600 km north from Helsinki. In Helsinki 5062 suicides were committed by men, and 2160 by women, whereas 903 by men and 278 by women in Oulu. The Finnish Meteorological Institute http://www.fmi.fi/ provided us with the daily data on sunshine hours and temperature, measured within the 25-km radius from these cities throughout the study period. For the day to day analysis, the daily sunshine, measured in minutes per day, (hereafter S) and the daily temperature, measured in degrees in Celsius and averaged as the daily mean value, (hereafter T) were compared with those on the previous day and changes were marked as (+) indicating an increase, and (-) indicating a decrease from the previous day. Thus, we ended with four types of days according to weather changes, coded as T+S+, T+S-, T-S+, and T-S-, concerning the data from Helsinki and Oulu regions.

In order to take into account the differences in the yearly population sizes within and between Helsinki and Oulu, the daily means of suicides were calculated into daily means of suicide mortality rates (suicides per 100,000), for men and women, per each year, and for both cities (Tables 1 and 2). Furthermore, in order to control for the different lengths of each period studied, and to avoid the bias of having dominance of certain type of weather changes within any period of the year, the daily mean of suicide mortalities (number of suicides per day, with 95% confidence intervals [CIs]) was calculated for each period in separate (Tables 3, 4, and 5).

Table 3

Astronomical seasons and men's (M) and women's (W) suicide mortality

Area Selected Days Astronomical season
Winter Spring Summer Fall
M Finland All .099
.094-.103		 .120
.115-.125		 .117
.112-.122		 .106
.101-.111
Helsinki All .128
.120-.136		 .138
.128-.148		 .135
.123-.147		 .131
.122-.140
T+ S+ .123
.099-.146 		.135
.122-.147 		.139
.122-.156 		.137
.120-.155
S- .113
.092-.134 		.144
.125-.163 		.142
.120-.164 		.115
.099-.132
T- S+ .122
.108-.136 		.146
.128-.163 		.130
.117-.143 		.133
.118-.147
S- .122
.097-.146 		.140
.120-.161 		.129
.112-.146 		.144
.116-.172
Oulu All .119
.098-.140		 .141
.123-.160		 .159
.138-.180		 .131
.113-.150
T+ S+ .107
.050-.163 		.134
.103-.165 		.162
.112-.212 		.152
.077-.227
S- .097
.059-.135 		.126
.082-.170 		.175
.132-.217 		.147
.101-.193
T- S+ .124
.094-.154 		.167
.132-.203 		.156
.115-.197 		.117
.073-.161
S- .097
.045-.148 		.138
.110-.166 		.132
.086-.178 		.144
.083-.205
W Finland All .025
.023-.027		 .031
.030-.032		 .029
.028-.031		 .028
.027-030
Helsinki All .046
.042-.050		 .050
.045-.054		 .049
.045-.054		 .048
.043-.053
T+ S+ .038
.028-.048 		.056
.046-.065 		.049
.039-.058 		.056
.043-.069
S- .051
.033-.069 		.049
.041-.056 		.046
.034-.058 		.052
.042-.062
T- S+ .042
.036-.049 		.046
.034-.057 		.049
.040-.057 		.049
.039-.059
S- .052
.039-.065 		.052
.042-.062 		.051
.041-.061 		.045
.031-.059
Oulu All .036
.029-.044		 .040
.030-.049		 .042
.032-.051		 .039
.029-.048
T+ S+ .032
.006-.058 		.031
.016-.046 		.042
.023-.061 		.049
.013-.086
S- .037
.015-.058 		.030
.015-.043 		.047
.020-.074 		.020
.005-.036
T- S+ .041
.019-.063 		.051
.028-.074 		.037
.019-.055 		.043
.019-.067
S- .028
.000-.055 		.045
.020-.069 		.042
.020-.063 		.047
.018-.076

Daily mean of suicide mortality and confidence interval of the mean in aggregate over the years from 1969 to 2003 for men and women, during astronomical seasons in Finland, Helsinki and Oulu, and according to daily changes (+/-) in temperature (T) and sunshine hours(S) in Helsinki and Oulu.

Winter: 21.12-20.03

Spring: 21.03-20.06

Summer: 21.06-22.09

Fall: 23.09-20.12

Table 4

Accelerating and decelerating periods of the equation of time and men's (M) and women's (W) suicide mortality

Area Selected Days Periods of time of equation
X1 Y1 X2 Y2
M Finland All .109
.105-.114		 .124
.118-.129 		.112
.107-.116		 .101
.096-.106
Helsinki All .131
.121-.141		 .142
.130-.154		 .130
.122-.139		 .130
.120-.140
T+ S+ .139
.116-.161 		.149
.131-.167 		.122
.097-.147 		.124
.097-.151
S- .151
.123-.179 		.145
.119-.170 		.124
.103-.145 		.110
.088-.131
T- S+ .152
.122-.182 		.126
.100-.153 		.135
.121-.150 		.120
.099-.141
S- .133
.104-.161 		.124
.106-.142 		.138
.116-.160 		.114
.086-.142
Oulu All .133
.115-.150		 .146
.124-.167		 .150
.131-.169		 .122
104-.140
T+ S+ .156
.094-.218 		.149
.101-.198 		.206
.122-.290 		.131
.052-.210
S- .096
.042-.150 		.207
.142-.272 		.165
.112-.217 		.097
.046-.148
T- S+ .199
.138-.259 		.140
.090-.190 		.138
.092-.184 		.115
.068-.162
S- .123
.065-.181 		.158
.114-.201 		.175
.107-.242 		.056
.008-.103
W Finland All .028
.027-.029		 .032
.030-.033 		.030
.029-.031		 .025
.024-.027
Helsinki All .047
.043-.051		 .053
.048-.058		 .050
.045-.054		 .043
.039-.048
T+ S+ .052
.040-.064 		.055
.046-.065 		.056
.044-.068 		.033
.018-.048
S- .044
.030-.057 		.059
.047-.072 		.055
.041-.069 		.055
.044-.067
T- S+ .051
.032-.070 		.049
.036-.061 		.049
.040-.058 		.036
.028-.045
S- .051
.034-.067 		.046
.034-.058 		.048
.037-.058 		.045
.025-.065
Oulu All .038
.029-.047		 .045
.034-.057		 .035
.027-.042		 .038
.029-.047
T+ S+
S-		 .023
.005-.042 		.039
.017-.060 		.044
.014-.074 		-
.035
.012-.058 		.053
.018-.087 		.015
.001-.029 		.026
.002-.050
T- S+
S-		 .062
.028-.096 		.030
.006-.054 		.042
.024-.061 		.036
.013-.059
.030
.003-.058 		.060
028-.091 		.032
.010-.054 		-

Daily mean of suicide mortality and confidence interval of the mean in aggregate over the years from 1969 to 2003 for men and women, during accelerating (X1, X2) and decelerating (Y1, Y2) periods of the equation of time in Finland, Helsinki and Oulu, and according to daily changes (+/-) in temperature (T) and sunshine hours(S) in Helsinki and Oulu.

X1: 11.02-14.05

Y1: 15.05-25.07

X2: 26.07-03.11

Y2: 04.11-10.2

Table 5

Advanced and delayed periods of the equation of time and men's (M) and women's (W) suicide mortality

Area Selected days Periods of time of equation
D2 A1 D1 A2
M Finland All .102
.098-.107		 .125
.120-.130		 .118
.112-.123		 .107
.102-.111
Helsinki All .128
.121-.136		 .146
.134-.158		 .135
.122-.147		 .130
.122-.139
T+ S+ .125
.103-.146 		.142
.129-.154 		.141
.123-.159 		.128
.108-.148
S- .123
.102-.143 		.142
.119-.164 		.145
.121-.169 		.116
.100-.131
T- S+ .124
.111-.136 		.162
.137-.187 		.124
.108-.141 		.134
.121-.146
S- .123
.103-.144 		.147
.124-.171 		.126
.110-.142 		.141
.118-.164
Oulu All .125
.107-.144		 .145
.123-.167		 .148
.128-.168 		.139
.119-.159
T+ S+ .099
.059-.139 		.159
.123-.196 		.137
.088-.185 		.186
.121-.250
S- .105
.065-.145 		.129
.080-.178 		.177
.129-.225 		.150
.104-.196
T- S+ .140
.112-.168 		.155
.103-.208 		.146
.095-.197 		.129
.090-.168
S- .109
.062-.157 		.137
.097-.176 		.122
.079-.165 		.149
.091-.206
W Finland All .026
.024-.027		 .032
.030-.034		 .030
.028-.031		 .028
.027-.030
Helsinki All .046
.042-.049		 .051
.045-.056		 .051
.046-.056		 .048
.043-.053
T+ S+ .043
.034-.053 		.058
.047-.069 		.048
.039-.057 		.056
.047-.065
S- .050
.033-.067 		.047
.033-.062 		.049
.038-.061 		.052
.042-.062
T- S+ .041
.034-.048 		.051
.036-.066 		.050
.040-.061 		.048
.040-.056
S- .048
.039-.058 		.057
.044-.069 		.051
.040-.062 		.048
.037-.059
Oulu All .036
.029-.044 		.043
.032-.054		 .041
.030-.053		 .038
.031-.048
T+ S+ .037
.016-.059 		.031
.013-.050 		.036
.017-.054 		.044
.019-.069
S- .033
.016-.049 		.033
.013-.054 		.058
.024-.092 		.017
.005-.030
T- S+ .045
.026-.063 		.047
.020-.075 		.040
.014-.066 		.040
.022-.058
S- .025
.002-.048 		.053
.021-.085 		.041
.019-.064 		.039
.020-.057

Daily mean of suicide mortality and confidence interval of the mean, in aggregate over the years from 1969 to 2003 for men and women, during advanced (A1, A2) and delayed (D1, D2) periods of the equation of time in Finland, Helsinki and Oulu, and according to daily changes (+/-) in temperature (T) and sunshine hours(S) in Helsinki and Oulu.

A1: 15.04-13.06

D1: 14.06-31.08

A2: 01.09-25.12

D2: 26.12-14.04

Finally, to rule out a potential confounder, we analyzed whether daylight saving time (hereafter DST) had any effect on the suicide mortality. DST was introduced in Finland 1981. From 1981 to 1994 DST lasted from the end of March until the end of September (hereafter DST1), and since 1995 DST has been in use from the end of March until the end of October, as in most parts of Europe (hereafter DST2). We calculated suicide mortality rates during one month period before, and after the transitions into and out of DST, separately for the years 1981 to 1994 (DST1) and years 1995 to 2003 (DST2), for which the suicide mortality rates of the corresponding periods during the years 1969 to 1980 were used as controls (Tables 6, 7, 8, 9,10, and 11).

Table 6

Men: Daily mean of suicide mortality, and switching into daylight saving time in spring.

Days 1969-80* 1981-2003
-1 m +1 m -1 m +1 m
M CI m CI m CI m CI
Finland All .097 .089-.106 .111 .102-.121 .104 .097-.111 .117 .110-.124
Helsinki All .114 .096-.133 .130 .114-.146 .135 .120-.150 .137 .118-.155
T+S+ .120 .061-.180 .160 .100-.219 .129 .084-.173 .140 .102-.177
T+S- .100 .054-.146 .156 .100-.212 .128 .084-.171 .181 .143-.220
T-S+ .123 .090-.156 .102 .067-.137 .139 .108-.171 .142 .100-.184
T-S- .144 .044-.243 .127 .061-.194 .180 .114-.246 .108 .080-.136
Oulu All .113 .064-.162 .114 .060-.168 .121 .092-.150 .158 .117-.199
T+S+ - - - - - - .110 .042-.179
T+S- - - - - .145 .060-.229 .146 .055-.237
T-S+ .123 .037-.208 .152 .013-.291 .104 .038-.171 .21 .139-.280
T-S- - - - - .065 .002-.129 .138 .039-.237

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch into daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Table 7

Women: Daily mean of suicide mortality and switching into daylight saving time in spring.

Location Days 1969-80 * 1981-2003(DST1 + DST2)
-1 m +1 m -1 m +1 m
m CI m CI m CI m CI
Finland All .024 .020-.028 .026 .023-.029 .027 .024-.030 .030 .027-.033
Helsinki All .055 .048-.062 .043 .031-.055 .043 .034-.052 .049 .040-.059
T+S+ .042 .014-.071 .052 .020-.083 .047 .019-.076 .055 .035-.076
T+S- .054 .028-.080 .031 .010-.052 .024 .012-.037 .046 .030-.062
T-S+ .061 .041-.081 .029 .004-.053 .040 .022-.057 .065 .028-.101
T-S- .052 .018-.087 - - .049 .020-.079 .044 .024-.065
Oulu All .033 .006-.059 .048 .016-.080 .048 .030-.066 .030 .009-.050
T+S+ - - - - - - - -
T+S- - - - - .053 .013-.094 - -
T-S+ - - - - .067 .018-.117 - -
T-S- - - - - - - - -

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch into daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Table 8

Men: Daily mean of suicide mortality, and switching away from daylight saving time in fall.

Location Days 1969-80* 1981-94 (DST1)
-1 m +1 m -1 m +1 m
m CI m CI m CI m CI
Finland All .111 .099-.122 .101 .094-.108 .119 .109-.128 .120 .112-.128
Helsinki All .141 .106-.177 .120 .104-.137 .139 .125-.153 .140 .122-.158
T+S+ .151 .064-.238 .115 .078-.151 .091 .039-.143 .163 .107-.219
T+S- .156 .076-.236 .107 .048-.166 .124 .087-.161 .123 .079-.167
T-S+ .137 .089-.185 .132 .083-.182 .156 .115-.197 .144 .107-.180
T-S- .138 .087-.189 .147 .085-.208 .184 .125-.244 .155 .062-.248
Oulu All .114 .060-.168 .125 .092-.158 .239 .165-.313 .121 .067-.175
T+S+ - - - - .324 .113-.535 - -
T+S- .100 .001-.200 .107 .002-.211 .256 .053-.458 .110 .000-.220
T-S+ .189 .084-.295 .130 .024-.237 .182 .060-.303 .103 .029-.176
T-S- - - - - .267 .142-.392 .229 .065-.393

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Table 9

Women: Daily mean of suicide mortality, and switching away from daylight saving time in fall.

Location Days 1969-80 * 1981-94 (DST1)
-1 m +1 m -1 m +1 m
m CI m CI m CI m CI
Finland All .028 .025-.032 .029 .026-.033 .029 .024-.033 .035 .031-.039
Helsinki All .052 .039-.064 .048 .033-.062 .046 .030-.061 .064 .049-.078
T+ S+ .071 .045-.097 .039 .019-.058 - - .074 .040-.108
S- - - .069 .033-.104 .049 .015-.084 .058 .033-.083
T- S+ .049 .025-.073 .055 .021-.089 .052 .031-.073 .067 .038-.096
S- .070 .022-.118 .023 .001-.046 .032 .005-.059 .065 .026-.103
Oulu All .033 .011-.055 .043 .006-.081 .020 .001-.040 .024 .001-.047
T+S+ - - - - - - - -
T+S- - - - - - - - -
T-S+ - - - - - - - -
T-S- - - - - - - - -

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Table 10

Men: Daily mean of suicide mortality, and switching away from daylight saving time in fall.

Location Days 1969-80* 1995-2003 (DST2)
-1 m +1 m -1 m +1 m
m CI m CI m CI m CI
Finland All .101 .094-.108 .101 .093-.110 .105 .095-.115 .097 .088-.105
Helsinki All .126 .108-.145 .140 .119-.161 .118 .079-.157 .106 .090-.121
T+S+ .116 .073-.159 .181 .078-.285 .128 .045-.212 .080 .033-.128
T+S- .134 .085-.182 .072 .017-.127 .117 .059-.176 .109 .075-.142
T-S+ .135 .079-.191 .177 .105-.249 .112 .071-.153 .093 .054-.131
T-S- .152 .072-.233 .204 .093-.314 .099 .035-.163 .104 .066-.142
Oulu All .131 .089-.173 .106 .058-.154 .145 .096-.195 .169 .107-.230
T+S+ - - - - - - - -
T+S- .104 .003-.205 - - .200 .069-.332 - -
T-S+ .167 .050-.284 .085 .001-.169 - - - -
T-S- - - - - .148 .006-.290 - -

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Table 11

Women: Daily mean of suicide mortality, and switching away from daylight saving time in fall.

Location Days 1969-80 * 1995-2003 (DST2)
-1 m +1 m -1 m +1 m
m CI m CI m CI m CI
Finland All .029 .026-.032 .025 .022-.029 .032 .026-.038 .027 .022-.032
Helsinki All .044 .029-.058 .042 .033-.051 .054 .039-.070 .035 .026-.044
T+S+ .032 .011-.052 .058 .019-.097 .104 .050-.159 .053 .002-.104
T+S- .055 .027-.083 .074 .036-.112 .049 .015-.083 - -
T-S+ .046 .015-.076 .032 .010-.054 .034 .002-.067 .040 .016-.065
T-S- .026 .001-.051 - - .064 .030-.099 - -
Oulu All .038 .005-.072 .032 .010-.054 .027 .004-.051 .051 .012-.090
T+S+ - - - - - - - -
T+S- - - - - - - - -
T-S+ - - - - - - - -
T-S- - - - - - - - -

Daily mean of suicide mortality during one month period before (-1 m), and after (+1 m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

The 95% CIs of the daily mean values, controlled for the length of a period of study and the male and female population sizes in a region of study, were used to evaluate the statistical significance, so that if they did not overlap with each other, it was judged to indicate a marked statistical significance.

Results

In Finland, during the years 1969 to 2003, the daily mean of suicide mortality was at the highest, with a statistical significance, for both men (mean = .124, CI = .118-.129) and women (mean = .032, CI = .030-.033), during the period Y1, i.e. from May 14th to July 25th , as compared to the nationwide references (Table 4).

Local photoperiod

The highest daily mean of suicide mortality seem to have emerged later in Oulu compared with Helsinki, but only for men. Therefore, the results of men are reported here in more detail. The daily mean of suicide mortality was at the highest during the period Y1 in Helsinki (mean = .142, CI = .130-.154, Table 4), but during the period X2 i.e. from July 26th to November 3rd in Oulu (mean = .150, CI = .131-.169, Table 4). The same postponed pattern was found also when the time pattern of suicide mortality was evaluated by seasons. The daily mean of suicide mortality was highest in Helsinki during spring (mean = .138, CI = .128-.148), but during summer in Oulu (mean = .159, CI = .138-.180). Furthermore, a similar postponed pattern was seen from A1 (Helsinki) to D1 (Oulu) periods (Table 5). However, these results did not reach statistical significance.

Local daily weather changes

For men, the days with T+S+ seem to have had the highest daily mean of suicide mortality both in Helsinki, during the period Y1 (mean = .149, CI = .131-.167), and in Oulu, during the period X2 (mean = .206, CI = .122-.290), which were the most "dangerous" periods in these cities. However, when estimated by the 95% confidence intervals, there was no statistical difference in the variation of means of suicide mortality between the four types of weather changes. The daily mean of suicide mortality in Helsinki and Oulu, however, do exceed the nationwide daily means of suicide mortality (mean = .124 for Y1 in Finland, and mean = .112 for X2 in Finland), as do all the underlined values for different types of weather changes in Helsinki and Oulu compared with each period at issue in Tables 3, 4, and 5.

Daylight saving time

The use or timing of daylight saving time did not have a significant effect on the suicide mortality (Tables 6, 7, 8, 9, 10, and 11).

Discussion

Nationwide results

Our key finding of statistical significance demonstrates the increased suicide mortality on nationwide level in Finland during the period from May 14th to July 25th. This 76-day period covers symmetrically both sides of summer solstice (Figure 1). During this period there is only 1 to 4 hours of darkness during the night in Helsinki but no darkness at all in Oulu. For the photoperiod dynamics in these locations, see http://www.gaisma.com/en/location/helsinki.html and http://www.gaisma.com/en/location/oulu.html, whose sunrise and sunset calculations are based on the algorithms displayed on National Oceanic and Atmospheric Administration Surface Radiation Research Branch web site at http://esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html, and e.g. for latitudes less than 72° north and south, accuracy is approximately one minute. It is of note here that the photoperiod in Finland due to its time zone is asymmetrical throughout the year, the period of daylight being always shorter for the a.m. hours than it is for the p.m. hours. This phenomenon influences the mechanisms that decode the duration of the melatonin signal in the melatonin-target tissues.

From the circadian-clock point of view, this period (May 14th to July 25th) is a challenge to alignment of the circadian rhythms with the sleep-wake cycle, and it resembles "the critical spring photoperiodic window" on intermediate to long days that has been characterized in sheep [37]. Some possible biological mechanisms for our current finding are briefly discussed in the following. The very long day (20 to 24 hours of daylight) might challenge the network within the circadian pacemaker that is comprised of the so-called evening and morning active cells, and that takes part in the seasonal adaptation in diurnal animals such as fruit flies [38,39] and sheep [37,40 - 42]. If this holds for humans as well, it is not known at the moment. If it does, it could mean that, when day lengths approximate fall and winter, the morning active cells dominate the circadian output, e.g. the sleep-wake behavior. This dominance of hierarchy is gradually transferred to the evening active cells as the days get longer in spring [38,39], the coincidence effect of the morning and evening active cells disappearing when the melatonin signal duration becomes insufficient to sensitize adenylate cyclase and to support a peak expression of the morning-active cells [37]. Interestingly, the speeding up of the evening active cells (e.g. by sunshine) makes the morning active cells run faster in long (summer) but not in short (winter) days [38]. In Finland, which is located at high to temperate latitudes with the light-dark transitions being most rapid around equinoxes, the asymmetrical photoperiod possibly favors the evening-active cells, and produces pronounced melatonin-dependent effects on gene expression during spring and fall. Whether such "locked morning active cells" contribute to the peak in deaths from suicide in spring in particular is not known. However, CRY2 and PER2 genetic variants, which might influence the evening and morning signals from the circadian pacemaker system, associate with depression vulnerability [43,44] in humans. Therefore, depressed individuals in particular might suffer from entrainment errors during periods that challenge the circadian pacemaker and predispose to circadian misalignment.

Local daily weather changes

The complexity of the circadian pacemaker system suggests that signals other than the seasonal changes in photoperiod, such as temporary variations in local weather conditions, are likely to play a role in the entrainment process [35,45]. Our finding of the later suicide peak in the northern area of study, Oulu-region, supports this. However, the daily mean of suicide mortality was almost as high also during the Y1 period in Oulu, as in Helsinki.

Hereafter we discuss the potential influence of daily weather changes for the suicide mortality in Helsinki and Oulu during the peak periods.

During the most dangerous periods, Y1 in Helsinki and X2 in Oulu, days with T+S+ seemed to be the worst for suicide mortality. From the circadian point of view the long daylight combined with the daily increase in ambient temperature and sunshine hours (T+S+) may have further phase advanced the circadian rhythm of the male suicide victims. An increase in sunshine hours and exposure to light may accelerate and advance the phase of the principal circadian clock, but an increase in ambient temperature and exposure to heat may have a similar effect [46]. The peaks of suicides have associated with ambient temperature in earlier studies [47 - 49], but so far, to our knowledge, the role of the circadian clocks has not been addressed.

Many lines of evidence suggest that abnormalities in the thermoregulatory processes are common among the depressed and therefore may cause or maintain the circadian misalignment. Patients with a major depressive episode tend to have elevated body temperature throughout the night, not during the day, and a phase advance of the circadian rhythm of core body temperature [18]. As hot nights might advance the phase of the circadian clock [50], and nocturnal body temperature during rapid-eye-movement sleep is influenced by hot, not cold, ambience [51], the dynamics of nocturnal temperatures might contribute to the advanced and rather fixed phase positions of circadian rhythms in major depressive episodes. In addition, sleep abnormalities, characteristically excessive rapid-eye-movement sleep at the cost of slow-wave sleep [17], are likely to give an abnormal (accelerating) feedback to the principal circadian pacemaker [26]. Further, during winter the duration of rapid-eye-movement sleep per night tends to increase [52], giving no support to deceleration and thereby favoring the desynchronization that may result in lowered mood and the subsequent increase in risk of suicide.

Daily fluctuations in temperature may play a part in the timing of suicides, either in combination with the long day length, or possibly also as a separate stressor. Studies concerning the over-activity in the functions of brown adipose tissue among the depressed [53] are most interesting in this respect, since the over-activity of brown adipose tissue may lead to reduced adaptation to rapid changes in ambient temperature that are typical during spring and fall. Once being activated, brown adipose tissue does not become quiescent easily [54], and if having been over-activated, it may through the thermoregulatory defect lead to disruption of the sleep-wake cycle and appetite control, and lead to early morning awakenings and loss of weight of the affected individual. Whether this kind of "vernalization failure" characterizes a suicide process and contributes to a mortality peak year after year is not known, but needs experimental data for analysis. However, in line with this background, for the Y1 period the daily mean of suicide mortality of men was at its lowest during the days of T-S- in Helsinki (mean = .124, CI = .106-.142, Table 4) and during the days of T-S+ in Oulu (mean = .140, CI = .090-.190, Table 4), suggesting that T- is a common nominator for the "safer" weather changes in both locations. T-S+ days were the "safest" also during the X2 period in Oulu. The daily decrease in temperature could therefore serve as a protective change during otherwise warm season. However, as the daily means did not differ significantly between the four types of weather changes, this is somewhat speculative thus far.

Limitations

Our limitation here is that we did not have the diagnostic information of the suicide victims, and that we demonstrate associations only, which do not necessarily tell anything about causality. Another limitation is that we did not have access to a suitable method, e.g. molecular-timetable methods [55] to be applied to a range of tissues, such as the brain and brown adipose tissue, from autopsy studies, to be able to analyze a mechanism of action and thereby to demonstrate a potential link between abnormalities in the circadian pacemaker system and death from suicide. On the other hand, our strengths include the nationwide sample of suicides for a long period of time, from a country with a high suicide mortality rate.

Conclusions

Our main findings here are that suicide mortality is higher during summer months and that daily changes in sunshine and ambient temperature are likely to modify the suicide mortality. Our findings presented herein now wait for tests by others in independent materials and is thus open to replication and the subsequent verification or falsification of the hypothesis. Some experimental data would be urgently needed for explanation of the mechanisms of action that take place in the brain of depressed patients and predispose them to suicide within those particular periods of time that we identified here. Suicide is a long process, whereas the timing of death from suicide appears far from random. In Finland from 1969 to2003 suicide mortality was elevated from May 15th to July 25th. This phenomenon should be considered also in clinical practice, since it bears implications for suicide prevention.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

Authors TP and KS designed the study and wrote the protocol. Author JL conceived and took part in designing the study. Authors LH and TP managed the literature searches and analyses and author LH wrote the first draft of the manuscript. All authors contributed to and approved the final manuscript.