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. 

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.

Phase-Shift Hypothesis

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. 

Georgetown University

Severe MDD with seasonal pattern can cause patients to seek psychiatric emergency room treatment. Studying emergency room visits shows that weather or seasonal changes characterized by low barometric pressure, rainstorms, shortened days, or high cloud cover results in a rise in emergency room visits for depression. Most of these weather patterns are more frequent in colder winter months, but MDD with seasonal pattern has also occasionally been reported during summer months.

Types of Weather that Aggravate 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

Low barometric pressure causes depression by disrupting the function of vestibular neurons, which are responsible for coordinating the body's balance and movement. When balance is disrupted, patients experience headaches and dizziness that can negatively impact their mood.

Low Barometric Pressure and MDD with Seasonal Pattern

Cloud cover blocks the sun's rays from reaching the Earth's surface. Exposure to sunlight stimulates serotonin production in the brain, and allows the body to synthesize vitamin D. During winter, shorter days and greater cloud cover can cause a drop in serotonin production and Vitamin D deficiency, which have both been linked to depression.

High Cloud Cover and MDD with Seasonal Pattern

Shortened Days and MDD with Seasonal Pattern

Studies have shown that in weather periods with heavy precipitation, interpersonal violence and conflict sharply increased among patients. While the exact mechanism behind this isn't known, theories suggest that crop shortages and food price hikes as a result of rainstorms or snowstorms may contribute. Given that precipitation is often heaviest in winter, these weather events may worsen MDD with seasonal pattern.

Heavy Precipitation and MDD with Seasonal Pattern

Seasonal MDD that flares up during summer months is less common than the typical winter MDD. The two types of seasonal MDD have notably different symptoms in affected patients. 

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