Springtime does not necessarily bring relief to people with depression. In fact, suicide rates around the world peak in spring.

On an average day, 105 Americans lose their lives to suicide. And counterintuitively, more of these lives are lost when the weather is warm and the sun shines bright.

Folk wisdom holds that winter is the most common time for suicides, with depressive symptoms exacerbated by cold, dark weather. Another myth suggests that suicides spike around the holidays, when struggling people feel left out of the cultural cheer.

In fact, studies dating back to the late 1800s find that suicides peak in the spring and are lowest in winter. One 1995 study published in the journal Social Science & Medicine examined monthly suicide rates in 28 countries and found that in 25 in the Northern Hemisphere, suicides were most common in May and ebbed in February. Similar findings occur in the Southern Hemisphere — in South Africa, for example, suicides peak in the southern spring, in September and October, according to a 1997 study in the journal Psychiatry Research.

The reason for this seasonality is unknown, but there are hints. Some researchers think weather or the ebb and flow of sociality drives the trend; others blame inflammatory processes that increase in spring. [Read More: Red Flags for Suicide & How to Help]

Suicide in America

There were 38,364 suicides in the United States in 2010, according to the most recent data from the Centers for Disease Control and Prevention (CDC). Those numbers put suicide as the 10th leading cause of death for all ages, and the third leading cause of death for people 15 to 24 years old. The topic of suicide was thrust into the headlines recently with the death of L'Wren Scott, a fashion designer whose March 17 death was ruled a suicide by police.

Women are more likely than men to say they have suicidal thoughts, but 79 percent of completed suicides in the United States are done by men. This is perhaps because men tend to choose more lethal means for suicide: About 56 percent of male suicide deaths are caused by firearms, according to the CDC. In contrast, the most common way for women to complete a suicide is by poisoning (37 percent of cases).

Predicting who will attempt or complete a suicide is a major barrier to prevention. Understanding why springtime is a more common time for suicide might help explain motivations, but even this question is tricky.

When the spring peak in suicides was first discovered in the late 1800s, the budding field of psychology cast around for explanations. One theory held the heat of spring and summer led to excess energy, which expressed itself in violence. Another theory suggested the intensity of social life in the spring and summer was to blame. [10 Controversial Psychiatric Disorders]

Socializing and suicide

A U.S. Army suicide prevention poster. The military is promoting suicide prevention in response to high rates of suicide among servicemembers. (Image credit: U.S. Army)

Some researchers still believe that weather may drive the increase in suicides in spring. It's possible that people who are depressed can't muster the energy to make and go through with a suicide plan in the winter, said Dr. Adam Kaplin, a professor of psychiatry and neurology at Johns Hopkins in Baltimore.

"Maybe spring comes and their moods are still down, and they get activated enough to kill themselves," Kaplin told Live Science.

The social explanation suggests people who are vulnerable to suicidal behavior face more challenges when the weather warms and social interaction increases. Alternatively, people who are struggling may feel left out of the increase in social engagements — essentially, they feel that the isolation of winter has withdrawn for everyone except for them.

Supporting these social theories, the 1995 survey of suicide across the globe found that the seasonality of suicide was greater in agricultural economies versus industrial ones. Agricultural countries are more seasonal, while indoor work tends to smooth out the effects of weather and sunlight on social life. Industrial countries show other small peaks, the researchers noted, particularly in the fall. These peaks may be linked to the start of the academic school year.

Likewise, researchers analyzed more than 50 years of suicide data from the Austrian penal system and found no evidence of seasonality in prison suicides, according to their 2004 study published in the journal Psychiatry Research.

"One of the core characteristics of penal institutions is the limited possibility for communication and social interaction," the researchers wrote. "This social isolation is independent of seasonal changes."

Thus, they reasoned, the influence of social opportunities on suicide rates is limited in prison. [5 Ways Relationships Are Good For Your Health]

Inflammatory theory

Social pressures aren't the only potential explanation for the spike in suicide in spring. Some researchers suspect that physical processes are to blame.

Depression, suicidal thoughts and behaviors have long been linked to inflammation, a chronic bodily response that occurs during times of stress. Inflammation is marked by an overactive immune system and dilated blood vessels, and it has been linked to chronic conditions ranging from asthma to heart disease.

The inflammation link to depression is well-established in studies in everything from mice to humans, Kaplin said. Mice injected with inflammation-triggering compounds stop grooming, socializing and working for sweets. Humans given the immune-boosting, inflammation-linked substance interferon-alpha to treat Hepatitis C are prone to becoming depressed as a side effect — in fact, Kaplin said, 25 percent of patients given the treatment develop clinical depression within weeks. And multiple sclerosis patients have the highest rate of depression of any medical, neurological or surgical disease, Kaplin said. Multiple sclerosis is an autoimmune disorder marked by inflammation in the brain and spinal cord.

In 2012, Michigan State University researchers found that increased levels of quinolinic acid, a compound generated by inflammation, in the spinal fluid were related to the level of suicidal thoughts a person had. The research was only in suicidal patients, not people who were depressed but not suicidal; but the findings suggest a way that bodily inflammation might affect the mind.

Some researchers see a link between this inflammation and springtime. In 2013, Danish researchers found an association between suicides and the amount of tree pollen in the air. The researchers analyzed 13,700 suicides in a population of 2.86 million Danes and found that suicides increased 6.5 percent when the pollen count reached 10 to 30 particles per cubic meter of air versus zero particles. A pollen count of 30 to 100 grains was linked to a 13.2 percent increase in suicides, the researchers reported in the journal BMJ Open.

Bolstering the inflammation link, suicide attempts also have been linked to air pollution levels, according to a 2010 study of emergency room visits for suicide attempts in Vancouver, Canada. Those findings appeared in 2010 in the journal Environmental Health Insights.

The findings of links between suicide and irritants in the environment are "provocative," Kaplin said, though the association is not yet proven. Clearly, more is at play than allergens, given demographic and cultural differences in suicide rates (In the United States, white men are most likely to kill themselves, and African American women least likely). But anti-inflammatory agents are already used in combination with anti-depressant drugs to boost their effectiveness, highlighting the biological roots of suicidal behavior.

"It's just letting people know [suicide] might not be from weakness. It might not be a character flaw," Kaplin said. "This might be a result of something of biological significance setting off a cascade that results in an illness setting off clinical depression that requires treatment."

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