By Lina Jamis

Snow-lovers, rejoice—winter is here in full force.

Compared to the summer months, the days are significantly shorter, which for some can mean the onset of the "winter blues." The medical term for this is (SAD), a condition caused by a mélange of changes in our biological clocks, serotonin levels, and melatonin production, all of which affect our mood.

While many of us are familiar with SAD, there are, in fact, people who get SAD in reverse. For a small group of people, the dark days of winter don’t elicit depression but renewed vigor and improved mood.

Reverse seasonal affective disorder affects less than one-tenth of all SAD cases, according to the National Alliance on Mental Illness. But just like winter-onset SAD, reverse seasonal affective disorder returns every year at about the same time.

While winter SAD is linked to a lack of sunlight, it is thought that summer SAD is due to the reverse—possibly too much sunlight, which also leads to modulations in melatonin production. Another theory is that people might stay up later in the summer, throwing their sensitive circadian rhythms for a loop. Interestingly, summer SAD and winter SAD seem to be prevalent in areas that are particularly prone to warmer summers. In other words, people in the southern U.S. tend to experience summer SAD more than those in the north, and vice versa.

Melatonin is a powerful and free radical scavenger that serves to protect the brain. More importantly, however, melatonin’s immediate precursor is the neurotransmitter serotonin, a major player in regulating mood. By reducing melatonin production, SAD increases the risk of depression and other mood disorders.

Research also suggests that high temperatures might play a role in reverse SAD. Notable differences between summer and winter SAD are that summer SAD individuals may typically feel , whereas those with winter SAD lack energy. Georgetown University psychiatrist and professor Norman Rosenthal, who first described and coined the term Seasonal Affective Disorder, notes that the drop in temperature can be calming for those people, who might otherwise find the summer heat oppressive and agitating.

Unfortunately, there are few studies devoted to understanding reverse SAD, likely because it is less well-known than its counterpart. In addition, individuals who might be affected by reverse SAD may be misdiagnosed with major depression, , or . Because it is fairly esoteric compared to winter SAD, many people who become depressed in the summer may not realize they have SAD. They may simply think of their bouts of depression as new events rather than parts of a pattern. Researchers think it may also have a genetic component; more than two-thirds of patients with SAD have a relative with a major .

So next time your friends decline an offer to go sunbathing or cavorting in the heat of July, consider that they might not actually be flaky but suffering from what hipster diva Lana Del Rey calls “Summertime Sadness.” As the next few months go by, those who experience reverse SAD will take comfort in knowing that the winter months can bring nothing but bliss with the gloriousness of gray skies, 15-hours of darkness, and bone-chilling winds.

On the other hand, I can be found inside. With a book. By a fireplace. Where it’s warm. Really warm.