What Question Were You Investigating with this Study?
Are there sex-based differences in exposure to light?
Light is an important environmental exposure, as it is the primary cue for the circadian system and has other effects on health. Yet there is limited objective evidence characterizing population-wide personal light exposure patterns.
What Approach Did You Use?
We analyzed real-world light exposure (using wrist-worn devices) collected from 11,314 participants in the 2011 to 2014 U.S.-based National Health and Nutrition Examination Survey, with metrics reflecting the duration of and the timing of exposure at different light levels (dim/low, moderate, and bright light).
What Did You Find?
The results show males spend approximately 52% more time in bright light than females, with this sex difference beginning in childhood and widening during adulthood. The average first daily timing for bright light exposure was also later for females, occurring approximately 30 minutes after males. In adults, time spent outdoors during workdays appeared to explain some, but not all, of the sex difference in the amount of time spent in bright light. These findings suggest that occupational factors may be playing a role in these sex differences, in addition to other factors.
While further research is needed, these findings suggest that these sex differences in light exposure may be due to sex differences in indoor vs. outdoor activities.
What are the Potential Effects of Low Light Exposure?
This analysis did not seek to evaluate the consequences of low light exposure, but we do know that light is the most salient cue for the circadian system and that light intensity, duration, and timing are some of the characteristics that can affect how the core clock (the suprachiasmatic nucleus) responds to light and relays information to other body clocks.
Dim or low light exposure will have a weaker effect on the circadian system than bright light, meaning that low light will not be as effective at synchronizing body clocks to the natural light/dark cycle. The timing of light exposure relative to the timing of the core body clock will also affect how light affects the body.
Poor synchronization of body clocks can adversely affect health outcomes such as the sleep-wake cycle and mood. Sunlight is also important for Vitamin D production, so limited daylight exposure (and greater low light exposure) could promote Vit D deficiency. There is also increasing interest in whether exposure to bright light during the day may be protective in buffering against the adverse effects of light at night (although further investigation is needed).
What's Next?
Future research on sex or gender differences in the response to light or light-related health outcomes should consider evaluating light exposure history and possible group differences in study design and analysis.
What Can People Do If They Are Not Getting Enough Exposure to Bright Light?
- Try to get daylight exposure upon waking (like taking a walk outside after waking up, having morning coffee outside, etc.)
- Try to increase overall daylight exposure by spending more time outside (taking breaks outdoors, having lunch outside, taking up an outdoor hobby, exercising outdoors, shifting an indoor meeting to be outdoors, designing spaces and environments that facilitate access and enjoyment of outdoor spaces, etc.)
- Try to increase daylight access and exposure in the indoor environment (using wall paints that reflect light, designing buildings that enhance daylight exposure, having easy access and proximity to windows in the indoor work or home environment, etc.)
Paper Cited: Wallace, D.A. Light exposure differs by sex in the US, with females receiving less bright light. npj Biol Timing Sleep 1, 16 (2024). https://doi.org/10.1038/s44323-024-00016-y
