Psychiatric News June 20, 2003
Volume 38 Number 12 Page 26
© American Psychiatric Association

• Clinical & Research News

Long, Sunny Days Linked To Suicide Incidence

1. Joan Arehart-Treichel

Bright sunlight—not temperature, rainfall, or barometric pressure—seems to explain why suicides peak in spring and summer, a new study from Australia suggests.

Now that the longest days of the year are here, at least in the Northern Hemisphere, many people are rejoicing. After all, sunlight is a tonic for most people’s mental health, especially for those who suffer from seasonal affective disorder.

Ironically, however, spring and summer seem to have a baleful effect on the psyches of other people. Both the severity of endogenous depression and the incidence of suicide peak in the spring and summer months. The evidence comes from several studies conducted in both the Northern and Southern hemispheres over the past quarter-century.

But what is it about spring and summer that triggers depression or suicide in certain people? It may well be exposure to bright sunlight, an Australian study reported in the April American Journal of Psychiatry suggests. The study links suicide not only with spring and summer, but also with the prevailing levels of sunlight.

Gavin Lambert, Ph.D., a research fellow at the Baker Heart Research Institute in Melbourne, Victoria, Australia, and his colleagues reasoned that if severity of depression and suicide incidence peak in the spring and summer, there must be some meteorological cue associated with these two seasons that alters brain activity and thus triggers depression or suicidal tendencies.

To identify that cue, the researchers looked at the frequency of suicide in the state of Victoria in the 1990s and whether suicide frequency could be linked with a particular meteorological factor.

Through the Office of the State Coroner of Victoria and the Victorian Institute of Forensic Medicine, they learned that 5,706 suicides had occurred in Victoria from 1990 to 1999. They obtained detailed meteorological data about the weather in Victoria from 1990 to 1999 from the Australian Commonwealth Bureau of Meteorology in Melbourne. The data included average daily temperatures, average daily atmospheric pressures, daily amounts of rainfall, and hours of bright sunlight daily.

They found that the incidence of suicide was highest in the spring and summer months and lowest in the winter ones. They then looked at whether there was any association between this seasonal pattern and seasonal variations in temperature, atmospheric pressure, rainfall, and bright sunlight. They found a correlation for bright sunlight, but not for the other factors. Seasonal fluctuations in suicides closely paralleled seasonal fluctuations in bright sunlight (see graph). This pattern was particularly marked for violent suicides.

Thus, “the incidence of suicide in southeastern Australia displays a clear seasonal pattern, being positively linked with prevailing levels of sunlight,” Lambert and his group concluded.

But why? Lambert told Psychiatric News that the neurotransmitter serotonin is probably involved, since he and his colleagues have found a link between bright sunlight and brain levels of serotonin in healthy human subjects and since some other researchers have found serotonin disturbances in the brains and cerebrospinal fluids of suicide completers. How serotonin might trigger suicidal behavior in certain people, however, is far from clear.

The investigation was funded by the National Health and Medical Research Council of Australia, the Wellcome Trust, and the Australian Rotary Health Research Fund.

The study, “Increased Suicide Rate in the Middle-Aged and Its Association With Hours of Sunlight,” is posted on the Web at http://ajp.psychiatryonline.org/cgi/content/full/160/4/793?. ▪

(2003) Am J Psychiatry 160, 793.

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