The production of the hormone melatonin, a sleep regulator, is inhibited by light and permitted by darkness. To some degree, the reverse is true for serotonin, which has been linked to mood disorders. Hence, for the purpose of manipulating melatonin levels or timing, light boxes providing very specific types of artificial illumination are effective.
Light therapy either uses a lightbox which emits a very bright 10,000 lux of light, much more than a customary incandescent lamp, or a lower intensity of specific wavelengths of light from the blue end (470 nm) to the green end (525 nm) of the electromagnetic spectrum.
We know that green light therapy at doses of 350 lux produces melatonin suppression and phase shifts equivalent to 10,000 lux bright light therapy and another study published in May 2010 now shows that cones also contribute to non-image light sensation, suggesting that the blue light now often used for therapy in depression or shift work should perhaps be replaced by green or white illumination.
Seasonal affective disorder
While full sunlight is preferred for seasonal affective disorder (SAD), light boxes may be effective for the treatment of the condition. The United States Food and Drug Administration has not approved the use of light boxes to treat SAD due to unclear results in clinical trials, but light therapy is still seen as the main form of treatment for SAD. Direct sunlight, reflected into the windows of a home or office by a computer-controlled mirror device called a heliostat, has also been used as a type of light therapy for the treatment of SAD.
It is possible that response to light therapy for SAD could be season dependent.
Light therapy has also been suggested in the treatment of non-seasonal depression and other psychiatric disturbances, including major depressive disorder, bipolar disorder and postpartum depression.. A meta-analysis by the Cochrane Collaboration concluded that "For patients suffering from non-seasonal depression, light therapy offers modest though promising antidepressive efficacy".
Circadian rhythm sleep disorders
In the management of circadian rhythm disorders such as delayed sleep phase syndrome, the timing of light exposure is critical. For DSPS, the light must be provided as soon after spontaneous awakening as possible to achieve the desired effect, as shown by the phase response curve for light in humans. Some users have reported success with lights that turn on shortly ''before'' awakening (dawn simulation). Morning use may also be effective for non-24-hour sleep-wake syndrome, while evening use is recommended for advanced sleep phase syndrome.
Light therapy has been tested for individuals on shift work, and for jet lag.
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