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Circadian Rhythms and Seasonal Affective Disorder

During the winter months, one of the most prevalent psychological disorders is the winter depression or, in medical terms, seasonal affective disorder (SAD). In the temperate regions, the days become shorter and the nights become longer as winter approaches and along with this, the symptoms of SAD gradually develop. SAD does not occur exclusively during the winter months but its development during summer is very rare.

Living things have an internal clock that dictates the timing and duration of most body processes and this is usually strictly followed to maintain the proper functioning of the body. The suprachiasmatic nucleus (SCN), acts as the master clock of the body and interestingly, it is found just above the optic chiasm, near the visual region of the brain. This may indicate that the body clock, also known as the circadian rhythm, is largely affected by visual input from the environment.

Circadian Rhythms and Seasonal Affective Disorder woman with winter depression

Women Are Found to Be More Prone to Seasonal Affective Disorder than Men

Seasonal Affective Disorder and the Effects of Light

Light is one key element that dictates the circadian rhythm of sleeping and waking. Once the SCN registers the information that the eyes have received light from the environment, it sends impulses to the sympathetic nervous system and then to the pineal gland to order the secretion of melatonin. The amount of melatonin released by the gland has an inverse relationship with the amount of light received by the eyes. Hence, during the winter months when the duration of the day is shorter, higher levels of melatonin are secreted into the bloodstream.

For people affected by SAD, stronger environmental signals are necessary to shift the circadian rhythm from the sleep state to the wake state and if the target point is not reached, the person may continue sleeping or may feel drowsy while awake. This is in line with the findings that patients with SAD tend to develop hypersomnia, sleeping excessively for several hours. This means that in SAD, the body may be experiencing problems with its ability to respond to light. Moreover, the disruption of the circadian rhythm due to discrepancies in the expected and actual input of light leads to confusion and discomfort, which can trigger the development of SAD.

Phototherapy for Seasonal Affective Disorder

Since light is one of the most influential factor necessary to banish SAD and normalize the circadian rhythm, phototherapy may be applied to SAD patients. In this therapy, a person with SAD is exposed to light with high intensity and broad spectrum for several hours a day in order to lower the amount of melatonin secretion as well as its amount in the bloodstream. Exposure to artificial light helps in bringing back the normal hormonal secretion pattern of the brain and other parts of the body, as well as in resynchronizing the circadian clock with nature’s 24-hour day-night cycle.

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