Biological Rhythms and the Body Clock Part II:

The Basis of Light Therapy for Seasonal Affective Disorder

The human body clock regulates not only daily rhythms such as sleeping and waking, but also seasonal changes in physiology. Humans that live in temperate zones where the climate changes with the seasons exhibit seasonal changes in behavior and activity just as other mammals do. The normal response of mammals to the changing seasons is to alter their activities and behaviors with respect to eating, sleeping, socialization, and sexual activity. Ref

It is the changing day length that signals the changes of the seasons to the internal body clock. In all mammals, including humans, information regarding the day length reaches the brain by way of a neural pathway from the eye to the biological clock in the brain. Ref In winter, the shorter days cause changes in the body's metabolism that can result in a depressed mood.

When seasonal changes of mood prevent normal functioning, this condition is known as Seasonal Affective Disorder (SAD) or seasonal depression. Light therapy in winter is used to make the days appear longer. This restores the summer pattern of sleep and metabolic activity, and restores the mood to a summer level.

One function of the body clock is to regulate the production of the hormone melatonin in the pineal gland. Melatonin has widespread influence on brain activity and influences serotonergic neural pathways, including those which affect mood. Studies suggest that light therapy can be used to alter the activity of these serotonergic pathways, and light therapy has accordingly been proposed, and used, as an alternative to treatment ith drugs for many forms of depression.
Read more on studies with light therapy for major depression and bipolar depression

Read more on the effect of aging on the body clock.

Part I: Light, Circadian Rhythms, and Sleep Disorders

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