Effects of a light therapy intervention on diurnal salivary cortisol in fatigued cancer survivors: A secondary analysis of a randomized controlled trial.

Objective: Altered diurnal cortisol rhythms are a potential mechanism through which symptoms of fatigue are maintained in post-treatment cancer survivors. Exposure to early morning bright light may target this underlying dysregulation, resulting in improved diurnal cortisol patterns, subsequently improving symptoms of fatigue. This research investigates the effects of a 4-week light therapy intervention on the diurnal cortisol slopes and output in fatigued cancer survivors.

Methods: Post-treatment adult cancer survivors who met diagnostic criteria for cancer-related fatigue were randomly assigned to receive either a bright white light (BWL) or dim red light (DRL) device, used daily for 30 min over four consecutive weeks. Assessments of fatigue and salivary cortisol were collected at baseline and post-intervention. Cortisol was sampled four times per day (waking, noon, 5 pm, bedtime) for three days at each timepoint. Diurnal cortisol slopes and total cortisol output were calculated at baseline and post-intervention. Linear mixed models were used to analyze the data.

Results: Seventy-seven participants were included in this analysis (BWL n = 40; DRL n = 37). Participants in both groups displayed increased steepness in cortisol slope (B = -0.02, p = .01, Cohen's d = 0.57) and increased total cortisol output (B = 9.58, p = .03, Cohen's d = 0.49) from baseline to post-intervention, indicating only a moderate effect of time. Neither diurnal cortisol slopes nor total cortisol output mediated the relationship between the light therapy intervention and fatigue levels.

Conclusion: Though the results of this trial are promising for light therapy as an effective intervention to reduce fatigue in cancer survivors, this does not appear to be achieved through alterations in neuroendocrine function. ClinicalTrials.gov registration #: NCT01780623.