Biological Rhythms and the Body Clock - Part II:

Light Therapy and Seasonal Affective Disorder (SAD)

The internal body clock of mammals regulates seasonal changes in physiology as well as the daily rhythms of sleeping, waking and metabolic activity. The seasonal alterations in the body clock of mammals triggers changes in metabolic activity that influences various behaviors which affect eating, socialization, breeding and other activities. As mammals, humans living in temperate zones also experience seasonal changes in metabolic activity that can affect their behavior. 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 human metabolism that can result in depressed mood.

Seasonal changes of mood can prevent normal functioning for many people. When this problem becomes severe it is termed Seasonal Affective Disorder, also known by its acronym SAD. Sad is defined as a major depression with a seasonal pattern, or seasonal depression. Light therapy in winter can be used to make the days appear longer. This restores a summer pattern of sleep and metabolic activity, and the mood reverts to that experienced in summer.

One function of the body clock in the brain is to regulate the production of the hormone melatonin in the pineal gland. Melatonin influences serotonergic neural pathways in the brain, 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 with drugs that affect serotonergic activity, such as SSRI's (Selective Serotonin Reuptake Inhibitors) which are used in the treatment of many forms of depression.
Studies with Lo-LIGHT therapy for non-seasonal major depression and bipolar depression.

MORE on The effect of aging on the body clock.

Part I: Light, Circadian Rhythms, and Sleep Disorders