Bright light therapy (BLT) refers to the use of bright light to treat symptoms of depression. BLT was initially prescribed as a treatment for patients with seasonal affective disorder.1 It was later found helpful for nonseasonal depression,2 premenstrual dysphoric disorder, postpartum depression, and phase shift circadian disorders, including for patients with dementia whose cognitive function improved after treatment with BLT.3 More recent studies suggest year-round benefit for nonseasonal depression.2 The American Psychiatric Association practice guidelines for the treatment of depression list BLT as an alternative and/or addition to pharmacologic and psychological treatment.4 BLT also may be beneficial for patients who are in the depressive phase of bipolar illness.
This article describes the evidence, rationale for use, mechanism of action, benefits, and safety profile of BLT for treating patients with bipolar depression.
Circadian rhythm disruption in bipolar disorder
Clinical manifestation. Patients with bipolar disorder (BD) spend more time in depression than in mania.5 Sleep disturbance is a core symptom of BD; patients typically have little need for sleep during a manic episode, and excess sleepiness during a depressive episode. Sleep complaints can be both precipitating factors and consequences of mood disorders. Patients with seasonal depression have excess sleepiness and weight gain in the winter followed by hypomanic-like symptoms in the spring, including decreased need for sleep and weight loss with psychomotor activation. In a recent review of sleep problems in patients with BD, Steinan et al6 reported that 20% of patients with euthymic mood in bipolar disorder experience a sleep disorder. Circadian disruption and “eveningness” (being more active during the evening) have been associated with mood episodes, functional impairment, poor quality of life, and treatment resistance.7-10
Pathophysiology. Existing hypotheses for the biological mechanism underlying dysregulation of circadian rhythm in BD include changes in melatonin levels, expression of melatonin receptors in the CNS, and daily cortisol profiles.11 Genetic evidence also links circadian rhythm dysregulation with BD. Two polymorphisms on the circadian locomotor output cycles kaput (CLOCK) gene that control circadian rhythm—aryl hydrocarbon receptor nuclear translocator-like (ARNTL) and timeless circadian clock (TIMELESS)—have been linked to lithium responsiveness in BD.12 In addition, Per2, Cry1, and Rev-Erbα expression—all components of the circadian clock—have been found to increase individual susceptibility to the therapeutic effects of lithium in mice.13 In addition, circadian rhythm dysregulation is associated with metabolic problems encountered by patients with BD, including weight gain, diabetes mellitus, and cardiovascular disease.14
Rationale for use
Regulation of a patient’s circadian rhythm disruption is a potential treatment for BD. Hashimoto et al15 demonstrated that midday bright light exposure can phase advance and increase the amplitude of nocturnal melatonin production in healthy individuals. Morning light therapy has been shown to increase blood serotonin throughout the day in both healthy individuals and in patients with nonseasonal depression; the effect was apparent with light intensities as low as 50 lux.16 Lithium may exert its therapeutic effect through its influence on the retino-hypothalamic-pineal tract and thus its effect on melatonin secretion.17
BLT is a logical choice to treat the depression phase of BD when exposure to sunlight is not feasible due to geographical location, season, or other factor. For patients who live in areas that receive frequent sunshine, an outside stroll for half an hour will likely achieve similar benefit to BLT.
The precise mechanism of action of BLT for bipolar depression has not yet been determined. It may be attributed to a phase-resetting effect via melanopsin and the suprachiasmatic nucleus (Box18-24).
Bright light therapy: How it works
The mechanism of action of bright light therapy is yet to be elucidated. The suprachiasmatic nucleus (SCN) in the hypothalamus is the center of circadian rhythm regulation and receives direct input from the retina through the retinohypothalamic tract.18 Melanopsin, a short-wavelength, light-sensitive G-protein–coupled receptor located in human retinal ganglion cells, is known to transduce short-wavelength light signals into neural signals.19 Since melanopsin is primarily responsible for resetting the timing of the SCN, suppressing pineal gland melatonin secretion and improving alertness and electroencephalogram-derived correlates of arousal,20 short-wavelength light with a low light intensity might be a better stimulator for melanopsin-containing retinal ganglion cells and the behaviors mediated via this photoreceptor system.21,22 Whether the antidepressant effect of light is also related to its alerting property is unclear.23 However, the acute alerting and performance-enhancing effects of light are increasingly taken into account for the design of indoor light standards in office environments.24 Response to light therapy is thus attributed to its phase-resetting effect.
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