Who Needs Bright Light and When? Mapping the Interactions of Lighting Environments and Individual Differences in Circadian Light Sensitivity.

Light is the primary circadian time cue, but there are large interindividual differences in how sensitive the circadian system is to light. Currently, it is not well understood how individual differences in light sensitivity interact with real-world light environments to determine sleep and circadian timing. We used a validated computational model to simulate sleep and circadian timing (predicted dim light melatonin onset) under realistic assumptions about light and work schedules. Simulations were repeated varying light sensitivity (translated to equivalent ED50 values for interpretability), as well as evening, morning, and daytime illuminances. Brighter evening light led to later predicted circadian and sleep timing, with this effect being amplified by high light sensitivity. Reducing evening light was particularly beneficial for those with high light sensitivity or a long circadian period. Brighter morning light was beneficial for individuals with a long circadian period, or those with both high light sensitivity and high evening light. However, bright morning light could be maladaptive in individuals with a short circadian period or those with low light sensitivity and low evening light. Brighter daytime light attenuated the delaying effects of evening artificial light across conditions, indicating that increasing daytime light was the most universally beneficial lighting intervention. Our results demonstrate how circadian light sensitivity can be used to tailor individual-level solutions that support optimal sleep and circadian timing.