Since the 1980's bright light has been used as an effective therapy for depressive and circadian rhythm disorders. Given at the right time of day, bright light can help those with seasonal depression and delayed sleep phase disorder. Interestingly, non-seasonal depression may also respond to this form of treatment when used in combination with antidepressant medication. For therapy to be effective, the light must be of appropriate intensity, used for the proper length of time and given at the correct time of day.

The greatest amount of research using bright light has been done on the treatment of seasonal depression. Bright light, or phototherapy, has also been used successfully to help people with circadian rhythm disorders. Seasonal depression may occur, at least in part, due to the difficulty people have in living in northern latitudes far from the sunny areas where humans originated - near the equator. Light therapy can help regularize the circadian system and help promote better mood and health. Circadian rhythm disorders may have some genetic basis but are also affected by the use of artificial lighting. Bright light long after the sun set is something that our ancestors who lived prior to the industrial revolution did not experience.

Light therapy has usually been delivered using bright light boxes. Originally these boxes were the size of a fluorescent ceiling unit - 2 feet by 4 feet. With a bank of full spectrum or cool white fluorescent lamps set about 1 meter (3 feet) away from the person using it, they provided 2,500 lux. Since these early units, smaller units have become available that are designed to reduce glare and to maximally filter ultraviolet light. These units can deliver about 10,000 lux when used at a distance of approximately 30 centimeters (1 foot).

Units should be professionally manufactured and not put together at home as it is possible to expose the eyes to excessive glare and dangerous ultraviolet frequencies. Use of incandescent lamps rather than fluorescent ones poses the additional risk of damaging the lens, cornea and retina due to infrared illumination, which makes up about 90% of the output of incandescent lamps. Any incandescent lamps marketed for use in light therapy should not be purchased. People with glaucoma or cataracts should use phototherapy only under the supervision of an ophthalmologist, as the effect of bright light on these disorders has not been well studied.

A main concern about the use of bright light is its effect on the eyes. The risks are damage to retinal cells, photosensitization to certain medications, and acceleration of macular degeneration. These risks are mitigated by proper use of well-constructed equipment.

After it has been determined that phototherapy is an appropriate treatment, the most complicated aspect is to accurately determine dosage and timing of treatment. This is usually accomplished in a close working relationship between the patient and the clinician. The therapy itself is self administered at home based on a schedule developed by the clinician. Adjustments may need to be made to the initial schedule and as improvement occurs. Sleep phase changes are usually tracked with sleep journals and patients with depression are often given symptom-rating scales to complete. Light therapy is usually administered in the morning based on the habitual waking time. Those with seasonal depression are usually given a dose of 10,000 lux for 30 minutes shortly after awakening. A similar strategy is used for those with delayed sleep phase but the dose may need to be increased to as much as 60 minutes. A challenge for those with delayed sleep phase is that they may need to awaken earlier than they have been and expose themselves to the bright light. This may be challenging for people who have been sleeping in but may be less difficult for those who have been forcing themselves to get up anyway for school or work.

Morning light therapy seems to work by advancing the circadian rhythm. That is, it helps people feel sleepy earlier and get up earlier. This seems to occur because the light exposure shifts the time at which melatonin is released in the evening. The degree to which bright light has an antidepressant effect depends on the degree of sleep phase advance that is achieved. Studies have shown that remission rates for seasonal affective disorder can approach 75 %. For non-seasonal depression, treated with light therapy alone, a remission rate of 50% has been reported.

Even though light therapy is generally very safe, negative effects can occur. Light therapy for depression and delayed sleep phase is typically given in the morning. If light therapy is given at night, it can cause insomnia and hyperactivity. If light is given too early, based on the patient's circadian rhythm, the sleep phase shift may be in the wrong direction and make the problem worse. Rare side effects that have been reported for patients with seasonal affective disorder include irritability, headache, hypomania and nausea. Some eye irritation has also been noted. These usually resolve after a few days of treatment but may require a change in the dose of light. At times it may be necessary to balance some mild side effects against the positive effects of treatment for seasonal or treatment resistant depression. Very rare cases of full mania have been reported in people with drug resistant non-seasonal depression. Unfortunately, in these rare cases light therapy must be stopped and the mania treated with medication. A very few cases of suicidal ideation requiring hospitalization have also been reported.

Light boxes can be obtained on the Internet and bright light therapy is generally very safe. Determining the appropriate dose and timing of light is more complicated and it is important to work with a trained professional rather attempting to treat oneself. Together, the person needing bright light therapy and their clinician can determine the most promising treatment schedule.

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