Seasonal affective disorder. Shedding light on a dark subject

Seasonal affective disorder (SAD) appears to be a disturbance of circadian rhythm caused by desynchronization between the solar clock and the human biologic clock during seasons of short photoperiods. The supplemental bright light of phototherapy resynchronizes the disturbed rhythm; however, a comprehensive theory to explain the mechanism of phototherapy is lacking. Future research on the action of melatonin and serotonin and the photochemical effect of light in relation to possible circadian rhythm disorders should help us to better understand and treat not only SAD but other conditions such as jet lag, premenstrual syndrome, eating disorder, and carbohydrate-craving obesity.