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The relationship between a lack of daylight and the depression associated with seasonal affective disorder (SAD) is not fully understood. However, SAD appears to be related to the way shifts in light change the seasonal activities of animals, including humans. Humans’ internal biological clocks – known as circadian rhythm – are altered during seasonal change, and in the winter months people may find that their biological clocks no longer correspond with their daily schedules. This may trigger depression.

In addition, the body hormone melatonin may play a role in the development of SAD. This hormone is produced at higher levels when a person is in a darkened environment – such as the shorter days and longer nights of winter – and it has been linked to higher rates of depression. Some experts also theorize that lack of sunlight leads to decreased production of serotonin, a neurotransmitter (brain chemical) that is involved in the regulation of mood.