Melatonin and seasonal affective disorder

In human, as in other mammals, melatonin is primarily a hormonal signal of darkness. One of the major interests in melatonin in the central nervous system is with regard to the possible mechanism of mood variations in response to light.

This phenomenon has been under intensive study in recent years and has been defined as syndrome called seasonal affective disorder.

In seasonal affective disorder recurrent episodes of depression occur during winter months and euthymia during summer season. These patients have delayed circadian rhythms.

In people with seasonal affective disorder, the photoperiodic hypothesis proposes that the period of melatonin release at night is longer in the winter than it is in the summer, whereas in people with seasonal affective disorder, how long the body is releasing melatonin each night does not differ across the season.
Melatonin and seasonal affective disorder