This hypothesis states that as a result of less sunlight exposure via shortened days and cloudy weather during winter, patients would experience excessive production of melatonin. Because melatonin causes feelings of fatigue and sluggishness, this excessive production would result in depression in the patient. This hypothesis has been mostly ruled out in favor of the phase-shift hypothesis. 

Georgetown University

Melatonin, a hormone responsible for regulating the sleep-wake cycle, is produced by the brain at night but suppressed by sunlight during the day. During the winter, earlier sunsets and later sunrises are thought to cause disruptions in the body's melatonin production schedule, resulting in seasonal depression. Two hypotheses to describe this disruption are the photoperiod hypothesis and the phase-shift hypothesis. 

Shortened Days and MDD with Seasonal Pattern

Cowen, P. J., & Browning, M. (2015). What has serotonin to do with depression?. World psychiatry : official journal of the World Psychiatric Association (WPA), 14(2), 158–160. https://doi.org/10.1002/wps.20229

World Psychiatry

Photoperiod Hypothesis

This hypothesis focuses on how disruptions to the body's circadian rhythm, driven by melatonin, affect the patient. During winter, as sunrise gets later, people begin waking up before dawn, so their body produces melatonin and drives their circadian rhythm later even though they are awake. For other people, an earlier sunset means their body produces melatonin too early before sleeping. Both of these situations can cause depression, fatigue, and sluggishness in the patient.

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