ADAA member Dr. Shane Owens contributes to this article on seasonal depression. “...Less light in the winter also means the body produces more melatonin (the hormone that regulates sleep), leading to chronic sleepiness and lethargy,” says Shane Owens, Ph.D., a New York-based board-certified behavioral and cognitive psychologist. On the flipside, a decrease in melatonin production in the summer (due to the increase in sunlight) may cause restlessness and insomnia.