Shining light on sleep regularity – daytime light exposure and nighttime sleep patterns

Kurzfassung

Introduction: Compelling evidence links irregular sleep and adverse health outcomes, yet these associations could be mediated not by sleep itself, but by co-occurring irregularity in other factors, including light exposure. We investigated the association between regularity of sleep and regularity of light exposure in the field, using continuous eye-level light recordings. Materials and Methods: Sleep/wake patterns were assessed for 30 days in 23 day-workers (61% females, range 21-38 years), using sleep diaries and actimetry. Spectral light exposure was recorded continuously at eye level for five days (including one weekend) at three wavelengths: blue (465nm), green (540nm), and red (620nm). Daily midsleep timing, timing of light exposure (acrophase), and intensity of light exposure (mean level) were extracted, and the Composite Phase Deviation (CPD) metric was used to calculate regularity scores for all three variables. The CPD uses vector lengths to measure how far away a given sleep/wake or light pattern is from a perfectly regular pattern. Hierarchical clustering was performed to identify groups of (ir)regular sleepers with (ir)regular light exposure. Results: Cluster analyses identified two main groups: irregular sleep/wake patterns were associated with irregular light exposure, and regular sleep wake/patterns with regular light exposure. Individuals in the irregular cluster were on average later chronotypes and had shorter sleep durations. High mean levels of light intensity only occurred in the regular cluster. The high mean light intensities resulted from irregular exposure to bright outdoor light on weekends, but not weekdays, revealing two distinct light profiles: exposure to occasionally high light levels (‘light showers’) vs. exposure to consistently low light levels (‘biological darkness’). Individuals with occasionally high light levels were on average earlier chronotypes, more regular sleepers, and slept longer, whereas the group with consistently low light levels included individuals with a wide range of sleep/wake behaviors (i.e., both regular and irregular sleepers). Conclusions: Our findings suggest that living in biological darkness (i.e., consistently low light levels) may allow other (physiological) factors, such as individual sensitivity to light, to shape sleep/wake behavior. Occasional exposure to bright light, such as spending weekends outdoors, may help improve sleep.