Light Exposure as a Digital Biomarker

Light exposure is the amount and timing of light reaching a person across the day and night. It matters because light is the dominant signal setting the circadian clock, and because modern life has compressed the difference between day and night: indoor daytime light is far dimmer than daylight, and evening light is far brighter than darkness.

It is unusual in this library in being an exposure rather than an outcome. Almost every other measure describes something the body is doing; light describes something being done to it. That makes it the natural upstream companion to the sleep and circadian measures, and the one modifiable input behind several of them.

It is also directly measurable. Where most wearable measures infer an internal state from an indirect signal, an ambient light sensor records the quantity of interest, which puts it on firmer footing than much of what sits around it.

Measurement uses a light sensor, either in a wrist worn device or in a dedicated research grade dosimeter, sampling illuminance continuously. Consumer implementations report time spent above a daylight threshold, while research devices record spectral content, which matters because the circadian system is most sensitive to short wavelength light rather than to brightness as the eye perceives it.

Consensus recommendations now exist for daytime, evening and night time indoor light exposure expressed in melanopic terms, which gives measured values something to be interpreted against.

The practical difficulty is sensor position. A wrist sensor is easily covered by a sleeve and points wherever the arm points, while the exposure that matters is at the eye. Wrist derived values systematically underestimate ocular exposure and do so inconsistently.

Research use has grown quickly. Large personal light exposure datasets have linked brighter nights and darker days to higher mortality risk, night light exposure to cardiovascular disease incidence, and personal light patterns to incident type 2 diabetes. Time spent in outdoor light has been associated with mood, sleep and circadian outcomes.

In intervention research, light is the exposure being manipulated in circadian and mood studies, and objective measurement is what distinguishes a delivered dose from a prescribed one.

It also serves as a covariate. Any study of sleep timing, mood or metabolic outcomes in free living participants has an unmeasured light exposure confound unless it measures it, and that is a stronger argument for collecting it than any single endpoint use.

Wrist measurement is a poor proxy for what reaches the eye. Sleeves, bags, pockets and arm position all attenuate the signal, and the resulting underestimate is not constant, so it cannot simply be corrected.

Consumer sensors report illuminance rather than spectral content, which means they cannot compute the melanopic quantities the consensus recommendations are written in. Values are also not comparable between devices, since sensors differ in spectral response and calibration.

There is no instrument in this library that measures light exposure, and the nearest counterpart, the Pittsburgh Sleep Quality Index, captures a downstream effect rather than the exposure itself. That is a real gap rather than an oversight, and it is why this measure is currently used as context rather than as an endpoint.

• Brown TM, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biol. 2022. pubmed.ncbi.nlm.nih.gov
• Windred DP, et al. Brighter nights and darker days predict higher mortality risk: a prospective analysis of personal light exposure. Proc Natl Acad Sci U S A. 2024. pubmed.ncbi.nlm.nih.gov
• Windred DP, et al. Personal light exposure patterns and incidence of type 2 diabetes. Lancet Reg Health Eur. 2024. pubmed.ncbi.nlm.nih.gov
• Burns AC, et al. Time spent in outdoor light is associated with mood, sleep, and circadian rhythm-related outcomes. J Affect Disord. 2021. pubmed.ncbi.nlm.nih.gov