A Cochrane review [Abstract] 1 included 20 studies with a total of 620 subjects. Most of the studies applied bright light as adjunctive treatment to drug therapy, sleep deprivation, or both. Daily duration or intensity of light did not influence the results. Treatment response, analyzed by primary mood rating scale endpoint scores and using a fixed effect model, was significantly better in the bright light group compared to the control treatment group (18 studies, 505 patients, standardized mean difference (SMD) -0.20, CI -0.38 to -0.01). This finding was mainly due to the significant benefit of short term treatment of seven days or less (12 studies, 367 patients: SMD -0.23, CI -0.44 to -0.02). In two short term studies that had applied bright light only, bright light was superior to control treatment (2 studies, 69 patients, SMD -0.64, CI -1.14 to -0.14).
Studies with a higher methodological quality rating showed unequivocal superiority of bright light over control treatment (2 studies, 50 patients, SMD -0.90, CI -1.50 to -0.31). The effect was weaker and statistical insignificant in studies with lower methodological quality (16 studies, 455 patients, SMD -0.12, CI -0.31 to 0.07).
Hypomania was more common in the bright light group than in the control treatment group (risk ratio 4.91, CI 1.66 to 14.46, number needed to harm 8, CI 5 to 20).
Comment: The quality of evidence is downgraded by imprecise results (limited study size for each comparison) and by limitations in study quality (e.g., lack of blinding, heterogeneity in interventions)
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