The circadian rhythm is more a function of darkness and light rather than actual time of day. Bright light can discourage drowsiness, and darkness can cause sleepiness, day or night. Light therapy is a treatment used for people who suffer from circadian rhythm sleep disorders. Your body has an internal clock that tells it when it is time to be asleep and when it is time to be awake.

This clock is located in the brain just above an area where the nerves travel to the eyes. This area is called the SCN. Your clock controls the “circadian rhythms” in your body. These rhythms include body temperature, alertness and the daily cycle of many hormones.

The word “circadian” means to occur in a cycle of about 24 hours. Circadian rhythms make you feel sleepy or alert at regular times every day. Some people have a circadian rhythm sleep disorder. This causes their natural sleep time to overlap with regular awake activities such as work or school.

Among other factors, your clock is “set” by your exposure to bright light such as sunlight. Exposure to bright light or “light therapy” is one method used to treat people with a circadian rhythm sleep disorder. The goal for treating patients who have circadian rhythm problems is to combine a healthy sleep pattern with an internal clock that is set at the right time. This will allow them to enjoy the benefits of good sleep.

Light therapy can help someone “re-set” a clock that is off. Regular sleep patterns help to keep the clock set at the new time. Light therapy is only part of a treatment plan that should be guided by a doctor who is familiar with sleep disorders. The use of a special light box may be helpful.

The procedure is noninvasive and simple. The patient sits a few feet away from a box-like device that emits very bright fluorescent light (over 4,000 lux) for about 30 minutes every day. The following people might benefit from light therapy in specific ways.

• Shift workers. Light therapy should be maximized during hours they are at work and minimized when they need to sleep.
• Frequent travelers. Light therapy may be useful for adjusting to new time zones and reducing jet lag.
• Nursing home patients.
• People with delayed sleep-phase syndrome. (These people have a natural tendency to fall asleep very late at night or in early morning hours, but then sleep normally.)

Everyone should check with their physician before using light therapy. The following people should avoid it or use it only under a physician's direction:

• Anyone with eyes or skin that is highly sensitive to light.
• Anyone taking medications that increase the risk for photosensitivity.
• People with bipolar disorder.

Timing of the therapy depends on the type of insomnia or sleep schedule of the individual. For example, in people who cannot get to sleep at night, light therapy in the morning and restricting bright light at night may be helpful. People who wake up early in the morning may benefit from light therapy performed in the evening, although a 2002 study reported that it had no effect in this group. Some light boxes have dawn/dusk simulators that help determine the correct brightness.

Patients typically receive bright light therapy at home, with the use of a light box. The light box emits a standard dosage of 5,000 to 10,000 lux (a measure of illumination) of white light while you sit in front of the light, at a specified distance, for approximately 30-60 minutes after waking in the morning. Light therapy should always be used within the proper limits for light intensity and duration of exposure.

Bright light therapy has not been known to show any major side effects. Some patients have reported minor side effects including: eye irritation and dryness, headache, nausea, and dryness of skin. To reduce the chance of experiencing these minor side effects, it is recommended that you begin light therapy very slowly and consult your doctor before use.