So you think good weather makes you feel good? That's what most of us think. Let's go to Florida or California, especially in the winter. The warm weather is the solution for the winter blues. Well, this is partly true. But there is a "dark" side to this sunny truth.
The whole concept of "seasonal affective disorder" is seriously misunderstood. People act like "SAD" is a new and separate condition from bipolar or unipolar mood illness. It really means seasonality in affective disorder. This idea has been well described for over a hundred years: you find it in Kraepelin's textbook, where he described clearly seasonal variation in mood, with more depression in the winter and more mania in the spring and summer.
This seasonality is not because of the temperature: it's not cold that makes you sad and heat that makes you happy. It's about light. Too little light triggers depression; too much light triggers mania. People with manic-depression are especially sensitive to light, and thus more likely to have these seasonal mood episodes. In fact, in the first modern studies of "SAD" in the 1980s, winter depression was accompanied by spring hypomania in 92% of subjects. In other words, "SAD" was the same thing as seasonality in bipolar type II illness.
Now to sunlight causing suicide: For over a hundred years, it has been noticed that the highest suicide rates state-by-state in the US occur in the sunny states of the mountain West and Southwest. Among those states, the highest rates are in the sunniest states.
Within states, like California, the highest suicide rates are in the sunniest parts, like San Diego County. These elevated rates have been high for a century, despite major changes in culture and economy (cowboys being replaced by techies); guns are common in the West, and the populations are sparse in some of those states, with low rates of psychiatrists per capita. Many have raised these other factors are reasons for higher suicide rates, and they certainly have a role.
Kraepelin had another theory: the sun causes suicide. Increased light will produce increased manic symptoms in persons with manic-depressive illness. Those increased manic symptoms often occur during mixed states (where depressive and manic symptoms co-occur), which is known to be the most dangerous mood state for suicide risk.
April is the cruelest month, T. S. Eliot wrote. Kraepelin proved it: April has the highest rate of suicide, with a major peak, because, he believed, patient experience a manic switch from depression to a mixed state as the spring light begins to increase after a long period of low light during wintertime. Kraepelin's classic charts have been confirmed in modern times, such as in the Danish study above. The effect of seasonality on suicide is pronounced especially in persons with mood illnesses.
The current study of the month (Vysokki et al, JAMA Psychiatry, 2014) confirms the theory of sunlight increasing the risk of suicide. All suicides in Austria from 1970 to 2014 were studied: 69,462 cases. Daily duration of sunshine was compared to daily number of suicide in different Austrian regions. A moderately strong correlation was found (r=0.49). After controlling for the season, the correlation was still there but fell to r=0.03, which is a small effect.
To learn more about high suicide rates in the American West over the past century, a classic source is Howard Kushner, American Suicide, Rutgers University Press, 1991.
For the original SAD paper where almost all 29 subjects had bipolar illness: NE Rosenthal et al, Seasonal affective disorder. A description of the syndrome and preliminary findings with light therapy. Arch Gen Psychiatry. 1984 Jan;41(1):72-80.