Examining photoperiod, chronotype, and adherence as predictors of morning light treatment effects on depression symptoms: A study of five clinical trials.

Morning light treatment (MLT) is increasingly being evaluated for a variety of health conditions; therefore, it is important to consider factors that may impact MLT outcomes. Natural light availability, chronotype, and treatment adherence have been posited to affect MLT. However, no prior studies have directly tested light availability and adherence as predictors of MLT outcomes and studies examining chronotype yielded mixed results. We aggregated data from five clinical trials of MLT (N = 86) for multiple conditions: probable posttraumatic stress disorder (NCT03513848), traumatic stress (NCT04117347), fibromyalgia (NCT03794908), human immunodeficiency virus, and chronic low back pain (NCT02373189). Depression symptoms are highly comorbid with all of these conditions; therefore, we examined whether light availability (season/photoperiod at MLT initiation), chronotype (self-reported morningness-eveningness and average bedtime prior to treatment), and objectively-measured MLT adherence predicted depression symptom change over time. All participants completed 60-min of daily MLT at home for 2 or 4 weeks using a light box or wearable device. We conducted a series of multilevel models nested by individual across time and study cohort. Depression symptoms improved across all trials. Later bedtime was a moderate but non-significant predictor of depression symptom improvement, providing preliminary evidence that later chronotypes may have better responses to MLT. Season and photoperiod were weak and non-significant predictors of depression symptom improvement, suggesting individuals may benefit from MLT throughout the year. Adherence also did not predict depression improvement, but these results should be interpreted with caution given that adherence was high and closely monitored in all trials.