 Dr. Lee says summertime sad could become even more common as global temperatures get more severe. Heat and excess light can induce fatigue and also mess with circadian rhythms for sleep. In the worst case scenario, it can also make people feel aggressive. The good news, she says, there are cures. Prioritize self-care, stay cool, literally speaking, and maintain a routine that focuses on healthy eating, regular sleep, and exercise. If the feelings still don't go away, she says it may be time to seek professional help.