Yes, a later bedtime is linked with a less active next day. In a Journal of Adolescent Health study published online 20 October 2026, 75 New Zealand young teens aged 11 to 14 wore activity trackers for 7 days. Each hour-later bedtime was linked with about 14 fewer minutes of moderate-to-vigorous activity the next day.
Key takeaways
- A repeated-measures study in the Journal of Adolescent Health (Taylor and colleagues, University of Otago, published online 20 October 2026) followed 75 young teens in Dunedin, New Zealand, for 7 days with wrist accelerometers, bedtime diaries and three 24-hour food recalls.
- Going to bed 1 hour later than usual was linked with about 14 fewer minutes of moderate-to-vigorous activity and about 4 fewer minutes of light activity the next day.
- On days with a later bedtime, the teens sat about 25 minutes more per hour of delay and ate about 150 more calories (618 kJ) from ultraprocessed foods.
- Activity during the day was not linked with sleep that night, and the authors found sleep timing more closely tied to diet and activity than total sleep time.
- The one change: give bedtime a fixed, earlier anchor and protect it on school nights and weekends alike, moving it 15 minutes at a time.
Does a late bedtime change the next day?
In this study, yes: a later bedtime was linked with a less active, more sedentary next day. Because each teen’s days were compared with their own other days, fixed differences between teens explain less of the pattern, but the links remain associations, not proof of cause.
Rachael Taylor and colleagues at the University of Otago measured 24-hour time use in 75 youths aged 11 to 14.9 (average age 12.9; 36% Indigenous Māori; 37% with overweight or obesity) between March and December 2021. Each wore an AX3 accelerometer for 7 days, wrote down bedtime in a daily diary, and completed detailed 24-hour dietary recalls on days 3, 5 and 7. Mixed-effects models compared each teen’s days with each other, adjusted for weekends.
| For each hour bedtime was later | Average difference |
|---|---|
| Moderate-to-vigorous activity the next day | about 14 minutes less |
| Light activity the next day | about 4 minutes less |
| Sedentary time | about 25 minutes more |
| Energy from ultraprocessed foods | about 150 calories (618 kJ) more |
| Energy from other non-core foods | about 175 calories (732 kJ) more |
| Energy from core foods (fruit, vegetables, grains, dairy, protein) | about 100 calories (419 kJ) less |
Does exercise during the day change sleep that night?
Not in this study. Light and moderate-to-vigorous activity during the day were not related to sleep that night, so the link ran mainly one way: from bedtime to the next day, not from the day’s activity to that night’s sleep.
That matters for where to start: an earlier, steadier bedtime may be a more practical lever than adding exercise and hoping sleep follows. The authors conclude that encouraging earlier bedtimes might be a practical target to reduce obesity risk, while noting it needs testing.
Why would bedtime matter more than total sleep?
The authors found that sleep timing, not total sleep time, carried the stronger links with diet and activity.
It fits earlier work. An earlier Otago study of 439 children aged 9 to 11 (the PEDALS study, Journal of Sleep Research, 2018) found that children who slept and woke late ate fruit and vegetables less often and got about 9 fewer minutes of moderate-to-vigorous activity a day than those who slept and woke early, even after accounting for sleep duration. At the SLEEP 2024 meeting, Penn State researchers reported that in 377 adolescents a later sleep schedule was linked with more carbohydrate intake and more sedentary time.
Adults show a similar pattern. A 2026 Medicine & Science in Sports & Exercise analysis of US national survey data (NHANES 2003 to 2006, 2,370 adults with sleep timing data) found that regular, early sleepers were about 14% more likely to meet physical activity guidelines than irregular sleepers.
What are the limits of the 2026 study?
It is small and observational: 75 teens in one New Zealand city over one week, with diet recalled on three days. Bedtime came from diaries, and a teen who stays up later may differ that night in ways the study did not capture, such as homework load, stress or a late event. It was not a trial of earlier bedtimes; that would be the stronger test.
What is the one change to make?
Give bedtime a fixed, earlier anchor and keep it within about an hour on school nights and weekends. For a young teen with a 7 AM wake-up, a 9:30 PM lights-out allows 9.5 hours, inside the 9 to 12 hours the American Academy of Sleep Medicine recommends for ages 6 to 12 (8 to 10 hours for ages 13 to 18).
- Start from the wake-up time and count back the sleep you need.
- Move bedtime earlier in 15-minute steps every few nights rather than all at once. See How to Fix Your Sleep Schedule: Anchor, Light, 15 Min.
- Close the evening with a short wind-down: pack the bag, lights low, a book.
- Keep dinner early enough that the kitchen closes well before bed. See Best Time to Eat Dinner? What the Evidence Says.
- Put active time where it fits the day, ideally outdoors in daylight. See Best Time to Exercise? What a 2026 Trial Found.
For families of older teens, school start times shape the morning side of the window: see Do Later School Start Times Help Teens Sleep More?. Curious when your own body clock runs best? Take the Body Clock Type quiz.
Your day on My BodyClock
The day above on My BodyClock shows a school day for a 12-year-old built around a 9:30 PM bedtime. Sleep runs from 9:30 PM to 7 AM, 9.5 hours. Breakfast and a bike ride to school lead into classes, then after-school soccer, a fruit-and-yogurt snack and homework. Family dinner is at 6:15 PM, followed by hobbies and family time. At 8:30 PM a short pink Flow block packs the bag and turns the lights down, and the last 45 minutes are a Relax block with a book before lights-out.
Set a bedtime block with its own alert and build your version in Owaves: My BodyClock on the App Store, or join the Android waitlist.
FAQ
Does going to bed late make kids less active?
In a 2026 study of 75 young teens in New Zealand, each hour-later bedtime was linked with about 14 fewer minutes of moderate-to-vigorous activity the next day. It is an association from one week of tracking, not proof that bedtime causes the change.
Do late bedtimes change what kids eat?
On days with a later bedtime, the teens in the 2026 study ate about 150 more calories from ultraprocessed foods and about 100 fewer from core foods such as fruit, vegetables and grains, per hour of delay. An earlier study of 439 New Zealand children found a similar pattern.
What time should a 12-year-old go to bed?
Count back from the wake-up time. The American Academy of Sleep Medicine recommends 9 to 12 hours for children aged 6 to 12, so a 7 AM wake-up points to a bedtime between about 7 PM and 10 PM, with 9:30 PM giving 9.5 hours. Teens aged 13 to 18 need 8 to 10 hours.
Will more exercise help a child sleep earlier?
In this study, daytime activity was not linked with sleep that night. Activity is valuable for its own sake, but the evidence here points to bedtime as the better place to start.
Sources
- Taylor RW, Haszard JJ, Wickham SR, Galland BC, Russell-Camp T, Jackson RF, Brosnan BJ, Meredith-Jones KA. Sleep, Dietary Intake, and Physical Activity: Examining Bidirectionality Using Repeated-Measures Analyses. Journal of Adolescent Health, published online 20 October 2026. doi:10.1016/j.jadohealth.2026.07.030. https://pubmed.ncbi.nlm.nih.gov/42900036/
- Harrex HAL, Skeaff SA, Black KE, et al. Sleep timing is associated with diet and physical activity levels in 9-11-year-old children from Dunedin, New Zealand: the PEDALS study. Journal of Sleep Research, 2018;27(4):e12634 (online 20 November 2017). doi:10.1111/jsr.12634. https://pubmed.ncbi.nlm.nih.gov/29160021/
- Sleep Review. Later Sleep Schedules May Derail Teen Nutrition and Activity Levels (SLEEP 2024 abstract, Penn State). 6 June 2024. https://sleepreviewmag.com/sleep-health/parameters/circadian-rhythms/later-sleep-schedules-derail-teen-nutrition-activity-levels/
- Zhang K, Lang JJ, Chen S, Colley RC, Saint-Maurice PF, Dumas A, Tomkinson GR, Chaput JP. Timing of Device-Measured Physical Activity and Sleep in Relation to Meeting Physical Activity Guidelines in U.S. Adults. Medicine & Science in Sports & Exercise, published online 28 September 2026. doi:10.1249/MSS.0000000000004154. https://pubmed.ncbi.nlm.nih.gov/42804660/
- American Academy of Sleep Medicine. Recommended Amount of Sleep for Pediatric Populations (consensus statement). 2016. https://aasm.org/resources/pdf/pediatricsleepdurationconsensus.pdf
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