Background: Abnormalities of the circadian rest-activity cycle are hypothesized to accompany the clinical picture of seasonal affective disorder (SAD). The purpose of this study was to investigate if bright light therapy (BLT) is able to reverse these disturbances.
Methods: Seventeen SAD outpatients and 17 sex- and age-matched healthy control subjects were treated with BLT administered in the morning for 4 weeks. Activity levels were measured with wrist actigraphy.
Results: SAD patients had 33% lower total (p = .031) and 43% lower daylight activity (p = .006) in week 1 compared with control subjects. The relative amplitude of the sleep-wake cycle was attenuated by 6% in patients (p = .025); they were phase delayed by 55 minutes (p = .023) and had significantly lower sleep efficiency (p = .030). Total (p = .002) and daylight activity (p = .001) increased after 4 weeks of treatment in SAD patients. Moreover, BLT led to increase of relative amplitude (p = .005), advance of delayed rhythms (p = .036), and improved sleep efficiency (p = .011) in patients. Intradaily stability, measuring the strength of coupling of the rhythm to external zeitgebers, increased by 9% both in patients and healthy control subjects (p = .032).
Conclusions: Treatment with BLT normalizes disturbed activity patterns and restores circadian rhythms in SAD patients. BLT might also stabilize the circadian rhythm in nondepressed individuals during the fall-winter season.