The seasons, particularly summer and winter, can be challenging times for people, according to Norman Rosenthal, M.D., a psychiatrist and researcher who led the team that discovered seasonal affective disorder (SAD, for short). Rosenthal is also the author of Defeating SAD (Seasonal Affective Disorder): A Guide to Health and Happiness Through All Seasons, which comes out in August.

Some people find themselves feeling depressed in winter, often due to a lack of sunlight during shorter days, but others feel depressed, irritable, and agitated in summer, Rosenthal says. 

When Rosenthal began researching seasonal affective disorder in the 1980s, he was initially focused on people who suffered during winters. But after hearing from many people who felt “the exact opposite” of winter SAD, his team started researching summer SAD too.

For people with summer SAD, feelings of depression and agitation begin to set in when it’s hot out, in May or June, and continue until the weather breaks around mid-September, he says. 

While winter SAD is often attributed partly to a lack of sunlight, Rosenthal says summer SAD could sometimes be triggered by an abundance of sunlight, which some people find agitating and which could negatively impact our sleep (which we know is important in regulating mood). The hot weather is also a likely culprit in summer SAD.

However, Rosenthal added that “Nobody knows for sure” what exactly causes summer SAD, and there could be other contributing psychological factors, like feeling worried that everyone else is having more fun than you are or insecure about shedding comforting winter layers and baring more skin. 

“If people are having this great time in the summer and you are not, you feel like you’ve been left out of some carnival that everybody else is participating in,” says Rosenthal.