Summary The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior however, evolved in the natural light-dark cycle[1] and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14h40min:9h20min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.

Summary Reduced exposure to daytime sunlight and increased exposure to electrical lighting at night leads to late circadian and sleep timing [1–3]. We have previously shown that exposure to a natural summer 14 hr 40 min:9 hr 20 min light-dark cycle entrains the human circadian clock to solar time, such that the internal biological night begins near sunset and ends near sunrise [1]. Here we show the beginning of the biological night and sleep occur earlier after a week exposure to a natural winter 9 hr 20 min:14 hr 40 min light-dark cycle as compared to the modern electrical lighting environment. Further, we find the human circadian clock is sensitive to seasonal changes in the natural light-dark cycle showing an expansion of the biological night in winter compared to summer—akin to that seen in non-humans [4–8]. We also show circadian and sleep timing occur earlier after spending a weekend camping in a summer 14 hr 39 min:9 hr 21 min natural light-dark cycle compared to a typical weekend in the modern environment. Weekend exposure to natural light was sufficient to achieve ~69% of the shift in circadian timing we previously reported after one week exposure to natural light [1]. These findings provide evidence that the human circadian clock adapts to seasonal changes in the natural light-dark cycle and is timed later in the modern environment in both winter and summer. Further, we demonstrate earlier circadian timing can be rapidly achieved through natural light exposure during a weekend spent camping.

Study Objectives Insufficient sleep is believed to promote positive energy balance (EB) and weight-gain. Increasing weekend sleep duration to “recover” from weekday sleep loss is common, yet little is known regarding how weekend recovery sleep influences EB. We conducted a randomized controlled trial to assess how: 1) 2 days and 8 days of insufficient sleep and 2) ad libitum weekend recovery sleep impact EB (energy intake [EI] – energy expenditure [EE]). Methods Following ten baseline days with 9h per night sleep opportunities, participants completed one of three 10-day experimental protocols with ad libitum EI: control (9h sleep opportunities; n=8; 23±5y [mean±SD]); sleep restriction (SR; 5h sleep opportunities; n=14; 25±5y); sleep restriction with weekend recovery sleep (SR+WR; 5 days insufficient sleep, 2 days ad libitum weekend recovery sleep, 3 days recurrent insufficient sleep; n=14; 27±4y). Results 24h EB increased (P < 0.001; main effect) by an average of 797.7±96.7 (±SEM) kcal during the 10-day experimental protocol versus baseline with no significant differences between groups. Percent change from baseline in 24h-EE was higher (P < 0.05) on day 2 of insufficient sleep (SR and SR+WR groups; 10±1%) versus adequate sleep (control group; 4±3%). Conclusions In this between-group study, the effects of adequate sleep and insufficient sleep, with or without or weekend recovery sleep, on 24h-EB were similar. Examining EB and body weight changes using within-subject cross-over designs and “free-living” conditions outside the laboratory (e.g., sleep extension) are needed to advance our understanding of the links between insufficient sleep, weekend recovery sleep and weight-gain.

Increased risk of obesity and diabetes in shift workers may be related to food intake at adverse circadian times. Early morning shiftwork represents the largest proportion of shift workers in the United States, yet little is known about the impact of food intake in the early morning on metabolism. Eighteen participants (9 female) completed a counterbalanced 16 day design with two conditions separated by ~1 week: 8 h sleep opportunity at habitual time and simulated early morning shiftwork with 6.5 h sleep opportunity starting ~1 h earlier than habitual time. After wake time, resting energy expenditure (REE) was measured and blood was sampled for melatonin and fasting glucose and insulin. Following breakfast, post-prandial blood samples were collected every 40 min for 2 h and the thermic effect of food (TEF) was assessed for 3.25 h. Total sleep time was decreased by ~85 min (p < 0.0001), melatonin levels were higher (p < 0.0001) and post-prandial glucose levels were higher (p < 0.05) after one day of simulated early morning shiftwork compared with habitual wake time. REE was lower after simulated early morning shiftwork; however, TEF after breakfast was similar to habitual wake time. Insufficient sleep and caloric intake during a circadian phase of high melatonin levels may contribute to metabolic dysregulation in early morning shift workers.

Introduction Timing of food intake has emerged as a novel risk factor for weight gain and obesity. Higher evening food intake, especially during insufficient sleep, is associated with weight gain. We aimed to explore initial changes in evening food intake and the respiratory quotient (RQ) during insufficient sleep and subsequent weight gain. We also explored sex differences. Methods 28 healthy adults (14F) aged 26.3±4.5y completed a 14–16 daylong laboratory protocol. In their home environment participants maintained one week of ~9h/night sleep schedules and consumed energy balanced diets for 3 days prior to completing the laboratory protocol. The laboratory protocol consisted of 3 baseline days of 9h/night scheduled sleep with energy balanced diets followed by 10 days of 5h/night scheduled sleep with ad-libitum food intake, with (n=14) and without (n=14) weekend recovery sleep. RQ was assessed on days 3 and 5 in a whole room calorimeter. Evening (dinner and after-dinner snacks) energy intake and body weight were assessed daily. Results A significant sex by condition effect was observed for evening food intake such that men and women were similar at baseline, but men ate more than women during insufficient sleep, when controlling for body mass (p&lt;0.05). A significant sex by condition effect was also observed for RQ with women showing similar RQ during baseline and insufficient sleep and men showing a higher RQ during insufficient sleep versus baseline (p&lt;0.05). Linear regression with food intake and RQ as predictors of weight gain showed that increased evening food intake, but not RQ, on the second day of sleep restriction was associated with weight gain in men, but not women, at the end of the study eight days later (p&lt;0.05). Conclusion Findings suggest that rapid changes in evening food intake during insufficient sleep contributes to subsequent weight gain during sustained insufficient sleep, especially in men. Support NIH HL109706, DK111161, TR001082, DK048520, Sleep Research Society Foundation grant 011-JP-16 and Office of Naval Research MURI (N00014-15-1-2809).