Lighting Protocols for Exploration: HERA Campaign

Introduction. Spaceflight exposes crewmembers to sleep loss and circadian misalignment, which impair cognition and increase the risk of errors and accidents. Light has both circadian resetting and direct alerting effects, therefore, we tested the effects of an experimental dynamic lighting schedule (DLS) on circadian phase, sleep, and performance during a 45-day simulated space mission.

Methods. Twenty participants [mean age (±SD) 38.7 ± 8.2 years; 7F] were studied in 4-person teams at the NASA Human Exploration Research Analog (HERA) Campaign 4 at the Johnson Space Center (JSC). Participants were scheduled to sleep for six 7-day cycles of 8 h/night for two consecutive nights then 5 h/night for 5 nights, ending with three 8-h recovery sleep episodes, aligned by waketime. We conducted a randomized between-subject trial to compare the effects of a dynamic lighting schedule (DLS) (n=8) with a static standard lighting schedule (SLS) (n=8). One mission (n=4) was excluded for light compliance failure. In the DLS, crewmembers were exposed to blue-enriched white light during all wake episodes (6500K) except for 3 hours before bedtime when the lighting was reduced in intensity and short-wavelength content (4100K, 2700K). Lighting was constant for all wake episodes in the standard condition. Circadian phase was calculated weekly from urinary 6-sulphatoxymelatonin (aMT6s) acrophase time, sleep was assessed using daily sleep logs completed before and during the mission, and cognition was assessed 3 times per week throughout the 45-day mission.

Results. The average (±SE) pre-mission aMT6s acrophases did not differ between the DLS (3.69 ± 0.38 h, n=8) and SLS (4.24 ± 0.38 h, n=8) conditions. The average acrophase was 1.55 ± 0.13 h later in the SLS compared to the DLS across the 45-day mission (p<0.05), with the largest difference being 1.79 ± 0.13 h on MD 33. The variability in acrophases across the 45-day mission was also significantly higher in the SLS (mean ± SD: 0.62 ± 0.06 h) than in the DLS condition (0.47 ± 0.04 h, p=0.04). The mean sleep duration on weeknights was significantly longer in the DLS (5.08 ± 0.04 h) compared to the SLS condition (4.95 ± 0.05 h, p=0.02), but similar between the conditions on weekend nights (7.50 ± 0.05 h vs. 7.30 ± 0.05 h, respectively, p=0.41). There was no overall difference in sleep onset latency (SOL) between conditions. Among the 10 cognitive tests, accuracy on the Abstract Matching and Matrix Reasoning Task (MRT) was significantly better in the DLS condition compared to SLS condition [p=0.020 and p=0.004 (FDR-adjusted p=0.039 and p=0.008), respectively].

Discussion. Compared to the SLS, the DLS significantly alleviated the drift in circadian phase typically observed in isolation analog studies, reduced the prevalence of sleep episodes occurring at an adverse circadian phase, and improved several mission-critical cognitive domains. Our results support incorporating DLS in future missions, which may facilitate appropriate circadian alignment and minimize the risk of sleep- and circadian-related decrements in performance.

The Earth-based application and commercialization potential of these studies are enormous. Electric lighting is ubiquitous and can improve alertness directly, thereby improving productivity and safety, and ultimately health. Anywhere where electric lighting is used, including schools, colleges, offices, factories, hospitals, care homes, residential homes, and transport and military applications, are potential beneficiaries. The International Space Station (ISS) ‘slam-shifts’ and the consequences to sleep, circadian rhythms, and performance, are very similar to that experienced by 15M American shiftworkers. Similar lighting interventions could be deployed for night shift-workers to improve productivity and safety while also reducing energy use. Many non shift-workers also suffer circadian misalignment, poor daytime alertness, and performance problems, and would also benefit from a dynamic lighting schedule.

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