Sunday, June 21, marked the summer solstice for the northern hemisphere, colloquially known as the first day of summer. Many sun-worshipers will revel in the longer daylight hours and warmer temperatures. But even after 50 years of Eddie Cochran's “Summertime Blues”—and a few of Lana Del Rey's “Summertime Sadness”—some people may be surprised to learn that summer can cause seasonal depression.

While seasonal affective disorder (SAD) is common during the short, cold days of winter, perhaps one in ten SAD sufferers experiences his or her depression during the summer months.

“Both summer SAD and winter SAD people can experience the full range of symptoms of major depressive disorder—depressed mood, hopelessness and feelings of worthlessness and nihilism,” says Ian Cook, a professor of psychiatry and bioengineering and director of the UCLA Depression Research & Clinic Program.

Other symptoms are opposites, like the seasons themselves. Winter sufferers often feel sluggish, sleep more than usual and tend to overeat and gain weight. By contrast, summertime depression often brings insomnia, loss of appetite, weight loss and feelings of agitation or anxiety. Summertime SAD can also create an increased feeling of isolation. If misery loves company, SAD sufferers can find plenty of other people to commiserate with during the dreary winter months. But during summer, most everyone else seems to be having a great time.

It remains a puzzle why some people experience SAD during the months of fun in the sun. Some research suggests that it can be triggered by too much sun exposure or oppressive heat. Other scientists have theorized that allergies play a roll, or that people are responding to shifts in sleeping habits during summer's lighter nights and bright early mornings.

Unfortunately for those with the summertime blues, winter SAD and other dangers of winter darkness have received the lion's share of research attention. “Treatments for summer SAD do not have as much evidence as there is for winter SAD,” Cook says. One common winter therapy, use of light exposure to help compensate for dark days, isn't likely to help those who become depressed during the long, bright days of summer. "Most clinicians take it case-by-case and empirically develop a treatment plan for each individual with summer SAD,” Cook adds.

So why does anyone suffer from SAD at all? A recent brain study suggests that the season in which someone is born may have a lasting impact on whether they are affected by the disorder. Researchers at Vanderbilt University pinpointed the mid-brain region that may be a source of SAD—the dorsal raphe nucleus, where many of the neurons that control serotonin levels are located. Serotonin is a neurotransmitter that helps to regulate mood; high levels have been linked to feelings of well being, while lower levels are associated with depression.

The team then raised groups of mice during different “seasons”, as defined by light cycles in the lab. Summer mice received 16 hours of light and 8 hours of dark each day. Mice representing spring and fall births experienced 12 hours of light and dark each. A third group, the winter mice, endured 16 hours of dark and only 8 hours of light each day. The groups' environments were identical in all other respects.

When the team recorded electrical activity in the animals' brains, they found that mice raised in summerlike conditions showed activity spikes consistent with serotonin secretion and elevated brain serotonin levels compared to their fall and winter counterparts—essentially, summer mice were happiest.

“The basic idea is that the enhanced activity of these neurons is a kind of antidepressant activity,” says Douglas McMahon, Vanderbilt’s Stevenson Chair in Biological Sciences, whose team reported their findings in May in Current Biology.

Brain changes due to seasonality were also reflected in mouse behaviors, the team found. Mice with brain chemistry consistent with that of a depressed human have been found to behave in certain ways. The forced swim test, for instance, is often employed to try out the effectiveness of antidepressant drugs. Scientists put mice into a pool of water and measure how much time they spend trying to escape versus just floating passively. Mice can float safely without much effort, but depressed mice, the theory holds, will more quickly lose hope of escape and simply float in despair. The Vanderbilt team ran this test with their mice, and the winter-born brood was quicker to float.

Similarly, an open field test determines how willing a mouse is to go out into the open. “You can imagine as a prey animal they are very cautious about that,” McMahon says. “But the mice born in summer were a bit bolder and less anxious, so they spent less time in closed areas or up against the wall,” he explains.

These birth season impacts lasted into adulthood for the mice, suggesting that the imprint of seasonal light on developing brains can stay with us even as we move around to different environments.

“We were able to show that their experience early on, even in what would be the equivalent of third trimester development in humans, sort of set the properties of the serotonin neurons, so that even six months later, and that's persisting into young adulthood for mice, they were still the same when we measured, even when we had moved them to live in a different seasonal photoperiod,” McMahon says.

He notes that researchers will still need to build evidence for this effect in humans. “In people, such an effect would have to persist for decades, and we don't know if it does,” he says. But other studies have also suggested that the season of our birth may make us more or less likely to suffer from various ailments—including depression. For example, earlier this month a Columbia University study compared 1,688 diseases with the birth dates of 1.7 million patients who had been treated at New York-Presbyterian Hospital/CUMC between 1985 and 2013. Among other ailments, several depression-related diagnoses were modulated by birth season, according to the study, with winter babies being more prone to suffer their effects.

“There could be lots of other seasonal variables other than light,” McMahon cautions. “But it's intriguing that at least in mice, our data shows a direct and lasting impact of the photoperiod on the neurons in the brain that are involved in producing serotonin and having an antidepressant role.”