Letting sleep shift a little with the seasons may be healthy. In a Sleep Health study of 357 Chinese adults, published in October 2026, each extra hour of seasonal variation in sleep length was linked with fasting glucose 6.5% lower a year later, while a 30-minute-plus weekday-to-weekend shift was linked with glucose 6.1% higher.
Key takeaways
- A 2026 Sleep Health study (Wang, Yang and colleagues, Anhui Medical University) followed 357 Chinese adults for one year, with three days of sleep diaries in each season and fasting blood glucose measured at the start and 1 year later.
- People whose sleep length changed more from season to season had fasting glucose about 6.5% lower a year later for every extra hour of seasonal variation. It is an association, not proof of cause.
- Weekly swings pointed the other way: a weekday-to-weekend shift of 30 minutes or more (social jetlag) was linked with fasting glucose about 6.1% higher than a shift under 15 minutes.
- In a 2017 Current Biology study, five adults camping near the winter solstice began sleep about 2.5 hours earlier and slept about 2.3 hours longer than at home under electric light.
- The one change: keep the same wake time every day of the week, and let the darker months move your bedtime earlier instead of sleeping in on weekends. The study did not test this step; it is a practical way to act on what it found.
What did the 2026 seasonal sleep study find?
Two kinds of sleep variability went in opposite directions. Sleep that changed with the seasons was linked with better blood sugar a year later, while sleep that swung between weekdays and weekends was linked with worse.
A team at the School of Public Health of Anhui Medical University in Hefei, China, asked 357 adults to keep a sleep diary for three consecutive days in each of the four seasons. From those diaries they worked out how much each person’s sleep length and timing moved across the year, and how far their sleep shifted between workdays and free days. Fasting blood glucose was measured at the start and again one year later.
| Kind of sleep variability | Linked with, over one year |
|---|---|
| Seasonal: sleep length varies more across the year (per extra hour) | fasting glucose about 6.5% lower |
| Weekly: weekday-to-weekend shift of 30 minutes or more (vs under 15 minutes) | fasting glucose about 6.1% higher |
| Seasonal variation in wake time | lower glucose only in carriers of one version of the MTNR1B melatonin receptor gene |
The authors concluded that keeping a natural degree of seasonal flexibility in sleep is associated with better glucose regulation, and that the link may depend on genes involved in melatonin signaling.
Is it normal to sleep more in winter?
Yes, it appears to be. When people live by natural light alone, the body’s biological night stretches in winter, and sleep starts earlier and lasts longer.
In a 2017 Current Biology study from the University of Colorado Boulder (Stothard, Wright and colleagues), five adults spent a week camping in Colorado near the winter solstice with no electric light. Compared with a week at home, their melatonin rise came about 2.6 hours earlier, they went to sleep about 2.5 hours earlier, and they slept about 2.3 hours longer (9.9 vs 7.6 hours), while wake time barely moved. Light during waking hours was more than 13 times brighter while camping than in their usual electrically lit routine. The authors also found the biological night was longer in winter than in summer. They noted that cold nights kept campers in the warmth of their tents, which may have added to the extra sleep.
Sleep stages seem to shift too. In a 2023 Frontiers in Neuroscience study of 188 patients at a Berlin sleep clinic who slept without alarm clocks, REM sleep was about 30 minutes longer in winter than in spring, and total sleep ran up to 60 minutes longer in winter than summer, a difference too small to be statistically clear. The patients had sleep problems, so the authors called for the work to be repeated in healthy people.
Why would weekend sleep shifts look worse than seasonal ones?
The study did not test why, but one likely explanation is that they answer different signals. A seasonal shift follows the sun, which sets the body clock, so the clock and sleep can move together. A weekend shift follows the social calendar, so sleep moves while the clock is still set by weekday light, and Monday can work like a small time-zone change.
That may be why the same study can link one kind of variability with lower glucose and the other with higher. The weekly result agrees with earlier work on social jetlag; see What Is Social Jetlag? What a 2026 Wearable Study Found. Modern evening light pulls the other way as well: Does Electric Light Make Us Sleep Later? A 12-Year Study looks at how lamps and screens push sleep later.
How does the clock change fit in?
In the United States, daylight saving time ends at 2 AM on the first Sunday of November, when clocks go back one hour (NIST). For many people that is when evenings suddenly feel long and dark, which makes it a natural point to let bedtime drift earlier for the winter. More on the date and what moves: When Does Daylight Saving Time End in 2026?
The extra hour on the clock works best as an earlier night rather than a later morning. Keeping the wake time fixed means the shorter days do the adjusting at the evening end, the way they did for the winter campers.
What are the limits of the study?
It shows links, not causes. Sleep came from three days of diaries per season rather than a wearable, the sample was 357 adults in China, and the follow-up lasted one year. People whose sleep flexes with the seasons may also differ in work, health or daylight exposure in ways the study could not fully separate.
The seasonal and camping studies are small, and none of them tested advice. Together they suggest that a modest winter shift in sleep is a normal response to shorter days, not a habit to fight.
What is the one change to make?
Hold your wake time steady seven days a week, and let the darker months move your bedtime earlier. Adults are advised to sleep 7 or more hours a night (American Academy of Sleep Medicine and Sleep Research Society, 2015), and winter is a fair time to give yourself more than that minimum. The 2026 study did not test this routine; it is a practical way to act on what it and the camping study found.
- Pick one wake time for every day of the week and keep it through the season.
- When evenings get dark early, move bedtime 15 to 30 minutes earlier rather than adding a weekend lie-in.
- Get outside soon after waking and again around midday, when winter daylight is brightest.
- Dim the lights and put screens away in the last hour before bed.
- Keep the weekday-to-weekend difference in sleep under 30 minutes.
Curious whether your body clock runs early or late? Take the Body Clock Type quiz. For how Chinese tradition marks the turn of the season, see When Is Lidong 2026, the Chinese Start of Winter?
Your day on My BodyClock
The day above on My BodyClock shows a winter weekday built around a fixed 6:30 AM wake time. Sleep starts at 10 PM, about half an hour earlier than a summer schedule, for eight and a half hours in bed. A daylight walk and breakfast open the morning, a walk outside after lunch catches the brightest light of the day, and the evening winds down with dinner, family time and a book under low lamps. The same wake time holds on the weekend.
Build your own winter week with a steady wake time in Owaves: My BodyClock on the App Store, or join the Android waitlist.
FAQ
Should you sleep more in winter?
Many people sleep a little longer in winter, and a 2026 study of 357 adults linked more seasonal variation in sleep length with fasting glucose 6.5% lower a year later. The study is observational, so it shows a link, not proof that sleeping more in winter causes better blood sugar.
Why do I feel sleepier when it gets dark earlier?
The body clock tracks daylight, and the biological night stretches when days are short. In a 2017 winter camping study, people living by natural light began their melatonin rise about 2.6 hours earlier and slept about 2.3 hours longer than at home.
Is sleeping in on weekends bad?
In the 2026 study, a weekday-to-weekend sleep shift of 30 minutes or more was linked with fasting glucose about 6.1% higher than a shift under 15 minutes. The study is observational, and it did not test fixes, but keeping wake time steady and adjusting bedtime is one practical way to get more sleep without a big weekend shift.
How should I adjust my sleep when the clocks fall back?
In the US, daylight saving time ends on the first Sunday of November and clocks go back an hour. Keeping your usual wake time and letting bedtime move earlier as evenings darken matches what the seasonal studies describe.
Sources
- Wang X, Cao Y, Sun Z, Liang S, Sun C, Gao J, Zhang Z, Yang W. Divergent associations of weekly and seasonal sleep variability with 1-year glycemic change in Chinese adults. Sleep Health, published online 10 October 2026. doi:10.1016/j.sleh.2026.09.008. https://doi.org/10.1016/j.sleh.2026.09.008
- Stothard ER, McHill AW, Depner CM, et al., Wright KP Jr. Circadian entrainment to the natural light-dark cycle across seasons and the weekend. Current Biology, 2017;27(4):508-513. doi:10.1016/j.cub.2016.12.041. https://pmc.ncbi.nlm.nih.gov/articles/PMC5335920/
- Seidler A, Weihrich KS, Bes F, de Zeeuw J, Kunz D. Seasonality of human sleep: polysomnographic data of a neuropsychiatric sleep clinic. Frontiers in Neuroscience, 17 February 2023;17:1105233. doi:10.3389/fnins.2023.1105233. https://pmc.ncbi.nlm.nih.gov/articles/PMC9981644/
- Watson NF, Badr MS, Belenky G, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep, 2015;38(6):843-844. doi:10.5665/sleep.4716. https://pmc.ncbi.nlm.nih.gov/articles/PMC4434546/
- National Institute of Standards and Technology. Daylight Saving Time. https://www.nist.gov/pml/time-and-frequency-division/popular-links/daylight-saving-time-dst
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