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

Introduction:
In post-comatose patients with disorders of consciousness (DOC), circadian (~24h) rhythms are often altered with the degree of disturbance being related to the patients’ clinical state. Light therapy has proven effective in ameliorating certain medical conditions (e.g., circadian rhythm sleep-wake disorders). However, its potential has not yet been evaluated systematically in patients with DOC, whose ‘light diet’ is usually characterised by fairly low illuminance during the day and relatively high illuminance during the night. Consequently, daytime melanopic light exposure is mostly insufficient for an optimal modulation of the circadian system. Thus, the aim of the present study was to investigate if indoor room lighting, which mimics natural daylight regarding its spectral composition and changes in illuminance, can re-entrain circadian rhythms in patients with DOC and consequently improve their clinical state.
Method:
We recorded skin temperature over 7-8 consecutive days in patients with unresponsive wakefulness syndrome (n=15) or minimally conscious state (n=2) in each of two light conditions. In the habitual light (HL) condition, patients were in a room with standard lighting. In the dynamic daylight condition (DDL), patients were in a room with ‘biodynamic’ lighting that was characterised by overall higher illuminance and dynamic variations in the spectral composition as they occur in natural daylight. To detect rhythmicity in the patients’ temperature data, we computed Lomb-Scargle periodograms (normalised power and period length), as well as interdaily stability and intradaily variability, which provide information about rhythm entrainment and fragmentation. For statistical analyses, we used advanced non-parametric statistical tests as implemented in the ‘nparLD’ package for R.
Results:
In the DDL compared to the HL condition, patients’ temperature rhythms deviated less from 24h (median deviation from 24h: DDL = 0.52h, HL = 3.62h), were more pronounced, more stable and less fragmented. Behaviourally, patients showed a stronger reactivity to external stimuli as indicated by a higher sum score during assessments with the Coma Recovery Scale-Revised in the DDL condition.
Conclusion:
Our findings indicate that adequate room lighting in intensive care units and long-term care facilities may be a promising and easy to realize therapeutic approach that helps to improve rhythm entrainment in severely brain-injured patients.