To determine the effect of light exposure on subsequent sleep characteristics under ambulatory field conditions.

Twenty healthy participants were fitted with ambulatory polysomnography (PSG) and wrist-actigraphs to assess light exposure, rest–activity, sleep quality, timing, and architecture. Laboratory salivary dim-light melatonin onset was analyzed to determine endogenous circadian phase.

Later circadian clock phase was associated with lower intensity (R² = 0.34, χ²(1) = 7.19, p < .01), later light exposure (quadratic, controlling for daylength, R² = 0.47, χ²(3) = 32.38, p < .0001), and to later sleep timing (R² = 0.71, χ²(1) = 20.39, p < .0001). Those with later first exposure to more than 10 lux of light had more awakenings during subsequent sleep (controlled for daylength, R² = 0.36, χ²(2) = 8.66, p < .05). Those with later light exposure subsequently had a shorter latency to first rapid eye movement (REM) sleep episode (R² = 0.21, χ²(1) = 5.77, p < .05). Those with less light exposure subsequently had a higher percentage of REM sleep (R² = 0.43, χ²(2) = 13.90, p < .001) in a clock phase modulated manner. Slow-wave sleep accumulation was observed to be larger after preceding exposure to high maximal intensity and early first light exposure (p < .05).

The quality and architecture of sleep is associated with preceding light exposure. We propose that light exposure timing and intensity do not only modulate circadian-driven aspects of sleep but also homeostatic sleep pressure. These novel ambulatory PSG findings are the first to highlight the direct relationship between light and subsequent sleep, combining knowledge of homeostatic and circadian regulation of sleep by light. Upon confirmation by interventional studies, this hypothesis could change current understanding of sleep regulation and its relationship to prior light exposure.

This study was not a clinical trial. The study was ethically approved and nationally registered (NL48468.042.14).