From Dawn to Dusk - Mimicking Natural Daylight Exposure Improves Circadian Rhythm Entrainment in Patients with Severe Brain Injury

Study Objectives While light therapy has proven effective in re-entraining circadian rhythms, the potential of such an intervention has not been evaluated systematically in post-comatose patients with disorders of consciousness (DOC), who often have strongly altered circadian rhythms. Methods We recorded skin temperature over 7-8 days in patients with DOC in each of two conditions: habitual light (HL), and dynamic daylight (DDL) condition. While patients were in a room with usual clinic lighting in the HL condition, they were in an otherwise comparable room with biodynamic lighting (i.e. higher illuminance and dynamic changes in spectral characteristics during the day) in the DDL condition. To detect rhythmicity in the patients' temperature data, we computed Lomb-Scargle periodograms and analysed normalized power, and peak period. Furthermore, we computed interdaily stability and intradaily variability, which provide information about rhythm entrainment and fragmentation. Results We analysed data from 17 patients with DOC (i.e. unresponsive wakefulness syndrome [n=15] and minimally conscious state [n=2]). The period length of the patients' temperature rhythms was closer to 24h in the DDL as compared to the HL condition (median deviation from 24h: HL=3.62h, DDL=0.52h). Specifically, in 11/17 (65%) patients the period length was closer to 24h in the DDL condition. Furthermore, the patients' rhythm was more pronounced, more stable, and less variable in the DDL condition. Conclusions Our results indicate that DDL stimulation entrains and stabilizes circadian rhythms. This highlights the importance of adequate room lighting as an adjunct therapeutic approach for improving circadian rhythms in severely brain-injured patients.