Light of different intensities and colors is used in conventional biomedicine and many nonconventional systems of medicine to treat both medical and psychiatric disorders. Diverse mechanisms of action are probably involved, including regulation of melatonin and neurotransmitters. Entrainment of sleep-wake cycles by external bright light cues and the associated suppression of melatonin production by the pineal gland is the established mechanism of action underlying the therapeutic benefits of light exposure on sleep and daytime fatigue.
Bright light therapy for depressed mood
Exposure to natural sunlight, especially in the early morning, has significant antidepressant benefits and may reduce the length of hospital stays in severely depressed patients who are hospitalized. A systematic review of controlled studies confirmed an antidepressant effect of bright light (10,000 lux) exposure therapy in seasonal depressed mood but provided only limited evidence supporting bright light as a treatment of nonseasonal depressed mood.
A more recent meta-analysis of controlled studies concluded that bright light exposure or dawn simulation for seasonal depressed mood and bright light exposure (but not dawn simulation) for nonseasonal depression had comparable efficacy to antidepressants. Findings of a small randomized controlled trial suggested that bright light therapy may be an effective alternative to antidepressants in pregnant depressed women.
Dim red or blue light for seasonal depressed mood
Recent studies suggested that regular exposure to dim red or blue light might be as efficacious as bright light, especially in the management of seasonal depressed mood. In a four-week single-blind study (57 subjects), patients diagnosed with seasonal affective disorder (SAD) were randomized to daily bright light versus dim red light exposure. Both groups experienced an equivalent and significant (40 percent) reduction in symptoms. Preliminary findings suggest that antidepressant effects of early-morning exposure to narrow spectrum blue or green light may be equivalent to benefits obtained from full-spectrum bright light exposure.
Exposing depressed patients to low-intensity artificial light approximately two hours before they would naturally be exposed to early-morning daylight increased the speed of response to conventional antidepressants. Thirty depressed inpatients treated with citalopram at 40 mg per day were randomized to early-morning dim green light, 400 lux, versus a sham, non–light-emitting device during the first two weeks of drug treatment. Patients in the combined citalopram–light exposure group reported significantly greater and more rapid improvements in mood compared to the citalopram only group. No adverse effects have been reported with regular exposure to dim light.
Light therapy for sleep disorders
An expert consensus report on light treatment for sleep disorders concluded that there is compelling evidence for the efficacy of bright light exposure in the management of circadian rhythm sleep problems but not for other types of insomnia. Most protocols recommend 30 to 40 minutes of bright light exposure daily for beneficial effects in shifting circadian rhythms and changing sleep–wake cycles.
Beneficial effects of bright light exposure therapy are usually noticeable within a few days, and an appropriate treatment plan often resynchronizes the patient’s sleep–wake cycle with his or her time zone in two to three weeks. Correct timing of light exposure is essential for successful treatment of circadian sleep problems, and bright light exposure between the hours of the late morning and late afternoon probably has no effect on circadian rhythm phase. Preliminary findings suggest that melatonin used in conjunction with bright light exposure may achieve more rapid normalization of the sleep–wake cycle.