Exposure to artificial light at night is widespread throughout the world, particularly in industrialized countries.
Since patterns of light and dark exposure play a key role in the timing of many behaviors and physiological functions, light exposure in the evening and night has been postulated to be detrimental to human health and well-being. . The impacts of light exposure during sleep are not as well studied as other types of nighttime light exposure.
However, a recent cross-sectional observational study noted that, compared with no light exposure during sleep, any self-reported artificial light exposure in the bedroom during sleep (small night light in the room, room light) outside the room or television/light in the room) was associated with obesity in women.
Additionally, the incidence of obesity was higher in those who reported sleeping with a television or light on in the bedroom. These findings suggest that light in the bedroom during nighttime sleep may negatively influence metabolic regulation.
Meaning
Exposure to nocturnal ambient light is implicated as a risk factor for adverse health outcomes, including cardiometabolic disease. However, the effects of nighttime light exposure during sleep on cardiometabolic outcomes and related mechanisms are unclear.
This laboratory study shows that, in healthy adults, one night of moderate light exposure (100 lx) during sleep increases nocturnal heart rate, decreases heart rate variability (increased sympathovagal balance), and increases resistance to insulin the next morning compared to sleeping in a dimly lit environment (<3 lx).
Furthermore, a positive relationship between increased sympathovagal balance and insulin levels suggests that sympathetic activation may play a role in the observed light-induced changes in insulin sensitivity.
Summary
This study tested the hypothesis that acute light exposure during nocturnal sleep negatively affects glucose homeostasis the following morning and whether this effect occurs through reduced sleep quality, suppression of melatonin or activation of the sympathetic nervous system (SNS) during sleep.
A total of 20 young adults participated in this parallel group study design. The room light condition (n = 10) included a night of sleep with dim light (<3 lx) followed by a night of sleep with superior room lighting (100 lx). The dim light condition (n = 10) included two consecutive nights of low-light sleep.
Measures of insulin resistance (morning homeostatic model assessment of insulin resistance, 30-minute insulin area under the curve [AUC] of a 2-hour oral glucose tolerance test) were higher under conditions ambient light than in dim light conditions. Melatonin levels were similar in both conditions.
In the room light condition, participants spent proportionally more time in stage N2 and less time in slow-wave rapid eye movement sleep.
Heart rate was higher and heart rate variability lower (higher sympathovagal balance) during sleep in the room light compared to the dim light condition. Importantly, greater sympathovagal balance during sleep was associated with higher 30-minute insulin AUC, consistent with greater next-morning insulin resistance.
- These results demonstrate that a single night of exposure to room light during sleep can affect glucose homeostasis, potentially through increased activation of the SNS.
- Attention to avoiding light exposure at night during sleep may be beneficial for cardiometabolic health.
