Light is an essential part of many life forms. The natural light–dark cycle has been the dominant stimulus for circadian rhythms throughout human evolution. Artificial light has restructured human activity and provided opportunities to extend the day without reliance on natural day–night cycles. The increase in light exposure at unwanted times or a reduced dynamic range of light between the daytime and nighttime has introduced negative consequences for human health. Light exposure is closely linked to sleep–wake regulation, activity and eating patterns, body temperature, and energy metabolism. Disruptions to these areas due to light are linked to metabolic abnormalities such as an increased risk of obesity and diabetes. Research has revealed that various properties of light influence metabolism. This review will highlight the complex role of light in human physiology, with a specific emphasis on metabolic regulation from the perspective of four main properties of light (intensity, duration, timing of exposure, and wavelength). We also discuss the potential influence of the key circadian hormone melatonin on sleep and metabolic physiology. We explore the relationship between light and metabolism through circadian physiology in various populations to understand the optimal use of light to mitigate short and long-term health consequences.

Ingesting oolong tea or caffeine acutely increases energy expenditure, and oolong tea, but not caffeine, stimulates fat oxidation. The acute effects of caffeine, such as increased heart rate and interference with sleep, diminish over 1–4 days, known as caffeine tolerance. During each 14-day session of the present study, 12 non-obese males consumed oolong tea (100 mg caffeine, 21.4 mg gallic acid, 97 mg catechins and 125 mg polymerized polyphenol), caffeine (100 mg), or placebo at breakfast and lunch. On day 14 of each session, 24-h indirect calorimetry and polysomnographic sleep recording were performed. Caffeine and oolong tea increased fat oxidation by ~20% without affecting energy expenditure over 24-h. The decrease in the respiratory quotient by oolong tea was greater than that by caffeine during sleep. The effect of oolong tea on fat oxidation was salient in the post-absorptive state. These findings suggest a role of unidentified ingredients in oolong tea to stimulate fat oxidation, and this effect is partially suppressed in a postprandial state. Two weeks of caffeine or oolong tea ingestion increased fat oxidation without interfering with sleep. The effects of subacute ingestion of caffeine and oolong tea differed from the acute effects, which is a particularly important consideration regarding habitual tea consumption.

Conventionally, women are perceived to feel colder than men, but controlled comparisons are sparse. We measured the response of healthy, lean, young women and men to a range of ambient temperatures typical of the daily environment (17 to 31 °C). The Scholander model of thermoregulation defines the lower critical temperature as threshold of the thermoneutral zone, below which additional heat production is required to defend core body temperature. This parameter can be used to characterize the thermoregulatory phenotypes of endotherms on a spectrum from “arctic” to “tropical.” We found that women had a cooler lower critical temperature (mean ± SD: 21.9 ± 1.3 °C vs. 22.9 ± 1.2 °C, P = 0.047), resembling an “arctic” shift compared to men. The more arctic profile of women was predominantly driven by higher insulation associated with more body fat compared to men, countering the lower basal metabolic rate associated with their smaller body size, which typically favors a “tropical” shift. We did not detect sex-based differences in secondary measures of thermoregulation including brown adipose tissue glucose uptake, muscle electrical activity, skin temperatures, cold-induced thermogenesis, or self-reported thermal comfort. In conclusion, the principal contributors to individual differences in human thermoregulation are physical attributes, including body size and composition, which may be partly mediated by sex.

Wearable sensors are commonly used to study the effects of free-living light exposure on physiological outcomes; however, rigorous validation of their performance has been limited. To address this gap, we quantified the accuracy and precision of light measurements from four commonly used wearable sensors (LYS, ActLumus, GENEActiv, and ActiGraph wGT3X-BT). Eight devices from each manufacturer (32 total) were compared to a criterion spectrometer under light-emitting diode–generated cool (peak ~460 nm) and warm (peak ~600 nm) white light at indoor intensities (2–1000 lux) and broad-spectrum sunlight (5 k–100 k lux). We found linear responses for all sensors between 50 and 20 k photopic lux, irrespective of spectrum, but detection ranges varied: ActLumus (2–100 k lux), LYS (2–60 k lux), GENEActiv (2–40 k lux), and ActiGraph (50–20 k lux). Accuracy—percent difference to criterion—also differed: ActLumus (+3.3 ± 13.4%, mean ± SD), LYS (+19.5 ± 42.0%), and below −20% for GENEActiv and ActiGraph. Interdevice variability was consistently lowest for ActLumus, and melanopic illuminance was less accurate with LYS than ActLumus, particularly in warm indoor light. Laboratory findings were compared to 1-week free-living light measurements in 21 individuals concurrently wearing the most and least accurate sensors in randomized positions on the nondominant wrist. The mean 24 h lux per participant was correlated (r = 0.91, p < .001) but lower for ActiGraph (77.3 ± 68.5 lux) than ActLumus (515.0 ± 436.0 lux; p < .001) with the greatest differences ≤100 lux (p < .001), consistent with the laboratory results. Thus, differences in illuminance range and accuracy can lead to large disparities in free-living measures across manufacturers, suggesting a need for greater technical standardization. Clinical Trial: A Natural History Study of Metabolic Sizing in Health and Disease. https://clinicaltrials.gov/study/NCT05398783 (NCT05398783). Statement of Significance Emerging evidence suggests light exposure has a greater impact on human health than previously appreciated. Wearable sensors embedded with light-measuring capabilities can play a crucial role in studying the relationship between light and physiological health. Thus, the performance of light sensors should be thoroughly evaluated. We tested the detection limits, accuracy, and variability of four commonly used wearable light sensors to indoor and outdoor intensities and spectra reflective of the real-world light environment and assessed the translatability of laboratory-based observations in a free-living pilot study. Our findings demonstrate broad variation in accuracy within and across manufacturers that emphasizes the need for rigorous sensor evaluation and highlights the challenges of interpreting illuminance measures across studies deploying different light sensors.

Research on mental health in working populations has predominantly focussed on negative outcomes, while studies on the positive aspects of life remain very limited, especially in Japan. Although morningness has recently been considered a factor for being happy, the role of sleep characteristics as it relates to the association between morningness and happiness has not been substantiated. The aim of this study is to investigate the associations between diurnal preference and level of subjective happiness in healthy, full-time, daytime workers in Japan. We also tested the moderation effect of sleep parameters on the relationship between diurnal preference and subjective happiness. This is an exploratory analysis from the cross-sectional data of the SLeep Epidemiology Project at the University of Tsukuba (SLEPT Study). Subjective happiness was evaluated using a single-item question. Diurnal preference was assessed using the Morningness-Eveningness Questionnaire (MEQ), in which higher scores indicate greater tendency to have morning preference. The participants underwent assessment of sleep parameters for 7 consecutive days using a waist-worn actigraphy device and kept a sleep diary. Sleep parameters investigated were subjective sleep quality, sleep disturbance, daytime sleepiness, weekend oversleep, total sleep time, sleep onset time, wake time, and sleep efficiency. A total of 205 males (average 42.6 ± 10.4 years) and 272 females (41.1 ± 9.8 years) were eligible for analysis. Hierarchical liner regression analysis was used to show the relationships of subjective happiness with MEQ score, and the sleep parameters. Further, moderation analysis was conducted by adding the interaction between MEQ score and the sleep parameters. After adjusting for age, psychological distress, self-rated health, and occupational stress, we found that subjective happiness was significantly associated with higher MEQ score and sleep efficiency but only in female. The moderating role of sleep parameters was not found. We discussed the implications of the obtained results and a possible strategy to maintain and improve subjective happiness of female workers who have evening preference.