Understanding Winter Seasonal Affective Disorder (SAD)

Medical Reviewer

There was a time when people spent most of their lives out of doors but today fewer than 10 percent of us see very much natural daylight, particularly during the winter months. Dark days, long nights and gloomy weather can result in a form of seasonal depression. Whether you call it the winter blues, winter depression or seasonal affective disorder (SAD) this form of natural light deficiency is officially recognized as a medical condition affecting millions of people every year.

People living in the northern hemisphere are most prone to winter SAD and, in general, the higher the latitude the more significant the risk of developing symptoms*.  These symptoms are relatively diverse in nature and may affect people in different ways. Perhaps the clearest symptom is a general feeling of depressed mood. Energy is lacking, drowsiness during the day is common yet sleep may be problematic at night. Cravings for high carb or sweet foods and drinks are higher and weight gain occurs. Some people feel more anxious, more irritable, and less capable. Social contacts may suffer and physical contact in terms of intimacy may also suffer.

Symptoms of depression may start as early as September and extend through the winter months until around April, with January and February often the worst months. Perhaps a third of the population is affected and many more experience milder 'subsyndromal' symptoms. Although we understand the importance of natural light and disruptions to the body clock that influence the development of SAD there are various explanations as to its cause and the reasons it affects some people and not others.

A family history of depression may increase the risk of SAD symptoms. Studies involving genetically identical twins for example, point to genetic considerations as increasing risk by much as 30 percent.

The hormone melatonin is released from the pineal gland in the brain during darkness. It controls both sleep and eating. One theory is that mammals and humans share the same basic mechanism. In mammals the longer nights increases the release of melatonin, which slows them down and/or prepares them for hibernation. According to the theory, most humans seem able to override the effects of melatonin but some appear more vulnerable to its effects and they experience a significant slow-down, as evidenced by the symptoms of SAD.

Others have suggested that the issue is less to do with the level of melatonin being produced and more the times it is secreted. Our natural body clock or circadian rhythm usually occurs over a 24-hour cycle. We know that 'normal' depression can result from poor sleep and in the case of SAD it is suggested that the transition from summer to winter changes the time that melatonin is released and this takes it out of alignment with other bodily rhythms.

Many people who suffer with depression may be familiar with the hormone serotonin. Serotonin transmits messages between nerve cells and one theory is that levels of serotonin are low in depressed people. Serotonin levels do seem to vary seasonally and less serotonin is produced with fewer hours of sunlight. Although some people do appear to respond to SSRI medications such as sertraline or fluoxetine, these appear less effective than light therapy, suggesting that depleted serotonin levels may only provide part of the picture.

Research into SAD has been ongoing for several decades and it could well be the case that other personality, social or psychological factors also have a role to play in the development of SAD. To date, treatment options include light therapy, antidepressant medication, cognitive behavioral therapy or other forms of counseling.


  • Rosen, L.N., Targum, S.D., Terman, M., Bryant, M.J. (et al) Prevalence of seasonal affective disorder at four latitudes. Psychiatry Research. 1990 Feb 3:31(2): 131-44.