The body clock coordinates daily (circadian) rhythms that the body experiences through the day. These rhythms include the cycle of waking and sleeping, the rise and fall of blood pressure and heart rate, and daily rhythms affecting gastrointestinal, renal, and immune system activity.   

The body clock aligns its timing to the external environment through exposure to light, normally sunlight. However, artificial light of an appropriate form can also be used. This Normally, very high intensities of artificial light intensity are needed to influence these rhythms.  However, the artificial light levels need not be bright, if appropriate light wavelengths of light are used.  Different wavelengths of light are perceived as different colors. 

Light therapy can be used to synchronize sleep patterns, in a manner similar to the wy sunlight normally synchronizes  the body clock with the external day and night. If the clock is stimulated by light before it expects the sun to have risen, it will reset to an earlier time.  If stimulated by light after the time it expects the sun to have set, the body clock will reset itself to a later time. 

Light therapy can be used to correct sleep phase disorders, which occur when a person's internal biological clock is delayed or advanced from the external environment. People with Delayed Sleep Phase Syndrome (DSPS) who are often younger, find it difficult to fall asleep until the early morning hours.   People with Advanced Sleep Phase Syndrome (ASPS) who are often older, tend to become tired in the late afternoon or early evening. 
Click for more information on aging and the body clock 

For people with a delayed circadian rhythm, the internal biological clock can be shifted to an earlier position and aligned with the external environment through exposure to appropriately timed light in the morning. Similarly, for people whose internal clocks are set too early, the internal clock can be shifted to a later time with evening light exposure.

However, the timing of light treatment with respect to the position of the internal clock is critical.  People with delayed or advanced sleep phase syndromes would be well advised to consult with a qualified sleep therapist before embarking on a program of light therapy.

The proposition by Sunnex Biotechnologies more than a decade ago that wavelength is as important as intensity in shifting the body clock has been confirmed by research groups around the world over the past 5 years. Independent university and military laboratories are now confirming that the most effective wavelengths for shifting the body clock are provided by the patented Sunnex Biotechnologies low-intensity Green LIGHT therapy technology. 
Click for more information on why the low-intensity Lo-LIGHT is as effective as bright light therapy 

The patented Lo-LIGHT technology comfortably shifts the internal body clock as effectively as bright white light.  Lo-LIGHT lamps screen out the hazardous  wavelengths of blue visible light. The Lo-LIGHT can therefore be used by people whose conditions make them sensitive to eye damage. This includes people with pre-existing ocular conditions, those with diabetes or other diseases that predispose them to retinal damage, or those taking photosensitizing medications or remedies like St Johns Wort.

PART II:  Seasonal Changes, Light Exposure, The Body Clock, and SAD.