insights

Read Time: 2 minutes

Environmental factors that disrupt the normal circadian cadence are known to have major impacts on health and well-being, one well-known example being the connection between shift-work and increased risk for a host of cardiometabolic health issues^1,2 and some cancers.³ It’s known that light exposure, one of the key zeitgebers for entraining circadian rhythms, plays an important role both in promoting beneficial effects, such as morning light boosting mood,^4.5 and in disrupting those rhythms, such as late-night blue-light exposure promoting poor sleep quality.⁵ Previous small and population-level studies have suggested that light exposure during the night may alter metabolic function, and now a large new prospective study has found evidence linking nighttime light exposure with risk of type 2 diabetes.⁶ The takeaway message from the study is that what is for many people a simple, low-effort, and low-cost lifestyle change, namely avoiding nighttime light, may have a large effect on diabetes risk.

While previous large studies that suggested a link between type 2 diabetes and light level used satellite data of outdoor light levels to estimate light exposure, the new study looked at personal light exposure by measuring it directly with a wrist-worn sensor. It is the first large-scale study to examine people’s individual light exposure patterns and their relationship with long-term health. Researchers used data from over 85,000 participants in the UK Biobank who wore light sensors for one week that recorded light from all sources (sunlight, artificial lighting, and digital screens). The researchers then tracked participants for an average of nearly eight years.⁶

They found that around half of the individuals were exposed to only about one lux of light or less during the night, defined in the study as 12:30 am–6:00 am, and this was used as the reference group. Compared to them, the researchers found a dose-dependent relationship between brighter light exposure at night and higher risk of subsequent type 2 diabetes. Specifically, compared to individuals with dark night environments (the 0-50th percentile), those in the 50-70th, 70-90th, and 90-100th percentiles of night light exposure had, respectively, 28-33%, 39-44%, and 53-67% higher risks for developing type 2 diabetes. For comparison, the increase in risk for the highest night light exposure group (the top 10%, who were exposed to around 50 lux) is about the same risk increase conferred by having a family history of type 2 diabetes, according to the researchers.⁶

This effect was unchanged when the researchers controlled for a variety of potentially confounding variables, including: age, sex, ethnicity, socioeconomic status, smoking, alcohol, diet, physical activity, urbanicity, daylight exposure, baseline cardiometabolic health, mental health, sleep duration, chronotype, photoperiod, shift-worker status, and existing pre-diabetes. The effect was the same for males and females, and they found it was also independent of genetic risk for diabetes as measured by polygenic risk score. The researchers noted that the difference in diabetes risk between people with bright and dark nights was equivalent to the difference between people with low and moderate genetic risk.

So how does light exposure at night potentially cause changes in insulin response and glucose metabolism? It might be tempting to suspect that sleep disruption is to blame as that itself is an established risk factor for type 2 diabetes.⁷ However, in the present study, night light exposure was an independent predictor of type 2 diabetes risk after adjustment for sleep duration, so it seems it may not be time spent sleeping that is a factor. Of course, that does not rule out sleep cycle changes that don’t impact sleep duration.

Another potential mechanism might be the basic disruption of circadian rhythms caused by mistimed light exposure, which could lead to metabolic dysfunction. As the researchers note, light exposure that suppresses or shifts circadian rhythms can alter both insulin and glucose secretion, shifting their timing relative to behavioral rhythms in nutritional intake, sleep, and physical activity. Changes in the pattern of melatonin or glucocorticoid release could lead to changes in insulin secretion and gluconeogenesis⁸ that coincide with food intake. Long term, this could lead to elevated glucose and promote insulin resistance⁹ and also inhibit pancreatic beta-cell function.¹⁰

Some shortcomings of the study include that the researchers did not have access to information about the timing of food intake, nor did they examine any personalized information about the individuals beyond the polygenic risk scores. The researchers note, for example, that light sensitivity can be highly variable, with the light intensity required to suppress 50% of melatonin secretion ranging from 6 to 350 lux across individuals.¹¹

Overall, the study found a robust dose-dependent relationship between light exposure at night and higher diabetes risk. The researchers note that this suggests that reduction of night light may be an easily implementable lifestyle recommendation that mitigates risk of diabetes, even in those with high genetic risk. Advising people to turn off their lights at night, to use lights that reduce the circadian impact (dim and warmer light vs. cooler, blue light), to use blue blocking filters on screens, or to wear blue blocking glasses are simple, low-cost recommendations that may help lower the risk of metabolic dysfunction and type 2 diabetes.

References

1. Vetter C, Dashti HS, Lane JM, et al. Night shift work, genetic risk, and type 2 diabetes in the UK Biobank. Diabetes Care. 2018;41(4):762-769. doi:2337/dc17-1933
2. Kervezee L, Kosmadopoulos A, Boivin DB. Metabolic and cardiovascular consequences of shift work: the role of circadian disruption and sleep disturbances. Eur J Neurosci. 2020;51(1):396-412. doi:1111/ejn.14216
3. Papantoniou K, Devore EE, Massa J, et al. Rotating night shift work and colorectal cancer risk in the nurses’ health studies. Int J Cancer. 2018;143(11):2709-2717. doi:1002/ijc.31655
4. Burns AC, Windred DP, Rutter MK, et al. Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nat Mental Health. 2023;1:853-862. doi:1038/s44220-023-00135-8
5. Siraji MA, Spitschan M, Kalavally V, Haque S. Light exposure behaviors predict mood, memory and sleep quality. Sci Rep. 2023;13(1):12425. doi:1038/s41598-023-39636-y
6. Windred DP, Burns AC, Rutter MK, et al. Personal light exposure patterns and incidence of type 2 diabetes: analysis of 13 million hours of light sensor data and 670,000 person-years of prospective observation. Lancet Reg Health Eur. 2024;42:100943. doi:1016/j.lanepe.2024.100943
7. Liu J, Richmond RC, Bowden J, et al. Assessing the causal role of sleep traits on glycated hemoglobin: a Mendelian randomization study. Diabetes Care. 2022;45(4):772-781. doi:2337/dc21-0089
8. Karamitri A, Jockers R. Melatonin in type 2 diabetes mellitus and obesity. Nat Rev Endocrinol. 2019;15(2):105-125. doi:1038/s41574-018-0130-1
9. Salans LB, Knittle JL, Hirsch J. The role of adipose cell size and adipose tissue insulin sensitivity in the carbohydrate intolerance of human obesity. J Clin Invest. 1968;47(1):153-165. doi:1172/JCI105705
10.  Javeed N, Brown MR, Rakshit K, Her T, Sen SK, Matveyenko AV. Proinflammatory cytokine interleukin 1? disrupts ?-cell circadian clock function and regulation of insulin secretion. Endocrinology. 2021;162(1):bqaa084. doi:1210/endocr/bqaa084
11.  Phillips AJK, Vidafar P, Burns AC, et al. High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proc Natl Acad Sci U S A. 2019;116(24):12019-12024. doi:1073/pnas.1901824116