The relationship between light exposure and sleep, fatigue, and depression in cancer outpatients: Test of the mediating effect

BACKGROUND:: Light is an important cue for the entrainment of circadian rhythms, which can be related to sleep quality, fatigue, and depression in cancer patients. OBJECTIVES:: The purpose of this study was to determine the effects of light exposure on sleep quality, fatigue, and depression in cancer patients, to test whether the effect of light exposure on sleep quality, fatigue, and depression was mediated by the other 2 symptoms. METHODS:: This was a cross-sectional study for which we recruited 163 cancer outpatients. For 3 consecutive days, they wore an Actiwatch to measure light exposure. Instruments included the Pittsburgh Sleep Quality Index-Taiwan form, the Brief Fatigue Inventory-Taiwan form, and the Beck Depression Inventory II-Taiwan version. RESULTS:: The results indicated that the Pittsburgh Sleep Quality Index-Taiwan form score was significantly and negatively correlated with minutes of light exposure (MLE) of 1000 lux or greater (r = -0.61, P < .001) and the intensity of light exposure under activity (r = -0.59, P < .001). Fatigue was negatively correlated with MLE (r = -0.18, P = .03). Depression was also negatively correlated with MLE and intensity of light exposure (both r = -0.18, P = .02). Most important, the effect of light exposure on sleep quality, fatigue, and depression was mediated by the other 2 symptoms. CONCLUSION:: Light exposure appeared to be a shared factor for the co-occurrence of fatigue, sleep disturbances, and depression. IMPLICATIONS FOR PRACTICE:: Light exposure has great potential for improving sleep quality as well as ameliorating fatigue and depression in cancer outpatients.