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Understanding how Bright Light
Affects Circadian Rhythm Disorders

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Note: This article is not intended as medical advice. One should always consult with a doctor when considering light therapy.

An Estimated 35 million Americans suffer from Seasonal Affective Disorder (SAD). Most scientists believe that SAD and other mood and sleep disorders are associated with shifted circadian rhythms (i.e. body clock). Those with ‘weak’ circadian rhythms need a very bright stimulus of light to reset their body clock each day. When they don’t receive this light signal, their body clocks either speed up (Advance) or slow down (Delay). This “Out of Whack,” body clock then produces the wrong hormones at the wrong time of day, causing the symptoms of SAD and other mood and sleep disorders. This article attempts to explain how bright light can re-shift delayed and advanced circadian rhythms back to their normal pattern.


The vast majority of people have no problem with sleep/wake cycles, but about 7% have a weak circadian system. Of this group, 80% will suffer from Delayed

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Sleep Phase Syndrome (DSPS), and need a very strong stimulus of bright light to reset their internal clocks each day. When they don’t get this stimulus, their body clock will migrate to a later time of day until it receives a strong enough stimulus of light.

The treatment of sleep disorders is based
on the hypothesis that the body's internal clock will only respond to bright light at a certain time of day.

Most people with sleep disorders fall into this category. They hate to get up in the morning, have a terrible time at school or work, and usually have trouble falling asleep until 2:00 or 3:00 in the morning. Their subjective morning has probably migrated to 12:00 p.m. or even later.

‘Peak Time’ Corresponds to Minimum Core Body Temperature
The treatment of sleep disorders with light therapy is based on the hypothesis that the body’s internal clock will only respond to bright light at a certain time of day. This ‘peak time’ for people with normal circadian rhythms occurs when they are in R.E.M. sleep, approximately 1 to 2 hours before awakening. Sleep labs can determine this peak time by one’s body temperature. When the body temperature drops almost a full degree, the pineal gland in the brain is most receptive to light. Light received just prior to this ‘peak time’ will delay the circadian rhythm and push the body clock forward, causing the person to wake up later, while light received just after will advance the circadian rhythm, causing the person to wake up earlier.

Normal circadian rhythm (waking time: 7:00 a.m.) Body temperature minimum usually occurs 1-2 hours before wakening




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