The summer solstice is June 20, the longest day of the year in the Northern Hemisphere. But do the extra hours of sunshine make you happier?

In general, people are more cheerful in the summertime, according to Philip Gehrman, associate director of the Behavioral Sleep Medicine Program at the University of Pennsylvania. Light serves as the strongest cue to regulate circadian rhythms, which include the sleep cycle, hormonal fluctuations and body temperature and follow a roughly 24-hour pattern.

“People tend to feel better in summer months,” Gehrman said. “There’s a slight elevation in our mood. More positive emotions are reported.”

But the effect isn’t uniform—circadian rhythms vary from person to person, so the amount and timing of sunlight means different things to different people, said Dr. Irina V. Zhdanova, a neuroscience professor at Boston University. For example, morning sunlight can irritate people who wake up late. On the other hand, for people who rise early, morning sunlight can have a positive effect, but sunlight may irritate them in the evening.