Light is a powerful synchronizer of the biological clock and of the sleep/wake cycle. Blind people have more sleep disturbances than people without eyesight problems. However, whether visually able people who are underexposed to bright natural light suffer from sleep wake disorders has never been examined. This study tried to assess the prevalence of sleep and wake disorders in subjects working in environments that are not exposed to natural light.
A setting-controlled cross-sectional epidemiological study was carried out. A representative sample of 13,296 French employees of a single transportation company participated in the study. During working hours, 4635 subjects (34.9%) experienced no light exposure (NLE) and 8661 were partially or completely exposed (LE) to natural light. Sleep disorders, sleep quality, and sleepiness were assessed using subjective tools: the Sleep Disorders Questionnaire-French version (SDQFV) and the Epworth Sleepiness Scale (ESS). Light exposure was estimated on workers' schedules and by objective measurements of light intensity (lux meter).
On a univariate analysis, complaints of poor sleep and sleepiness were significantly higher in NLE workers compared to LE: nonrestorative sleep (36.8% vs. 29.5%; P<.0001), insomnia (28.8% vs. 24.8%; P<.0001), severe insomnia (14.1% vs. 10.9%; P<.0001), and daytime somnolence (6.8% vs. 4.3%; P<.0001). After multivariate analysis, NLE has shown more insomnia (OR=1.8, 95% CI 1.3-2.3, P<.01) and hypersomnia (OR=1.9, 95% CI=1.3-2.4, P<.01) than LE.
These data suggest that underexposure to natural light at work may significantly impair sleep and wake disorders in non-light-exposed workers.