Teen sleep is easy to frame as a battle over phones, homework, streaming and one more scroll before bed. But a new study suggests sleep may matter for more than next-day mood and focus.
Researchers from the University of Copenhagen and the COPSAC research unit at Herlev and Gentofte Hospital found that longer sleep was associated with more stable blood sugar in healthy 18-year-olds. The study, published in Sleep, suggests that sleep and metabolic health may be connected earlier in life than many people assume.
The study followed 206 Danish 18-year-olds for two weeks as they went about their usual routines. Participants wore wrist activity trackers to measure sleep and movement, along with continuous glucose monitors to track blood sugar. In all, researchers analyzed 2,245 days of overlapping sleep and glucose data.
The pattern was two-directional. On days after longer sleep, participants had fewer blood sugar swings and fewer extreme glucose spikes. When participants had more stable blood sugar during the day, they also tended to sleep longer the following night.
“The more stable participants’ blood sugar was during the day, the longer they slept the following night,” said first author David Horner, a postdoctoral researcher from COPSAC at Herlev and Gentofte Hospital and the University of Copenhagen. “So this appears to be a two-way relationship — and that’s new.”
Blood sugar naturally rises and falls throughout the day in response to meals, activity, stress, sleep and the body’s internal clock. In people with diabetes or prediabetes, large or frequent swings can be a sign of metabolic strain. This study looked at healthy young adults, which makes the findings more subtle. The researchers were not studying disease, but early patterns that may help explain how sleep and metabolism interact.
“For most 18-year-olds, diabetes feels like something far off in the future. We’ve known very little about what blood sugar variability means for this age group,” said senior author Morten Arendt Rasmussen, a professor in the Department of Food Science at the University of Copenhagen and COPSAC. “But here we’re seeing similar patterns even in completely healthy young adults.”
The researchers also found that longer sleep was linked with slightly higher blood sugar levels in the morning and early part of the day. That was not what they expected, but they said it may reflect the body’s normal preparation for waking.
One possible explanation is the natural morning rise in cortisol, a hormone involved in the transition from sleep to wakefulness. Cortisol can affect blood sugar around waking, and the study suggests that this early-morning window may be important for metabolism.
“This may reflect the body getting ready for the day,” Horner said. “For example, the natural morning rise in the stress hormone cortisol can affect blood sugar around waking. Our findings suggest that the transition from sleep to wakefulness may be an important window for metabolism.”
Still, the study cannot show exactly why sleep and blood sugar were linked. Food choices, meal timing, physical activity, stress, screen use and daily routines may all play a role. The researchers also did not show that more sleep directly prevents obesity, type 2 diabetes or other metabolic diseases.
That matters because teen sleep is complicated. Bedtimes are shaped by biology, school schedules, homework, sports, jobs, social lives and phones that are designed to keep people engaged. The findings are not a reason to blame teenagers or parents for every late night.
They do add to a broader picture of sleep as part of metabolic health. For young people, getting enough sleep may support steadier daily rhythms in the body, including the way glucose is regulated. For families, that may make sleep routines worth treating as a health habit, not just a school-night rule.
“We already know that sleep is important for mental wellbeing. Our study adds to our understanding of why sleep is also crucial for physical health — and that this applies even early in adulthood,” Rasmussen said.
For now, the study offers one more reason to take teen sleep seriously. Blood sugar is often discussed as a matter of carbs, sugar or weight, but the body does not manage glucose in isolation. Daily routines, including sleep, may be part of the story long before chronic disease appears.
The study was supported by the European Research Council, the Novo Nordisk Foundation and the University of Copenhagen’s BRIDGE, Translational Excellence Programme. Researchers from COPSAC, the University of Copenhagen, the University of Washington School of Medicine, Queen’s University, Harvard Medical School, Steno Diabetes Center Copenhagen and Herlev and Gentofte Hospital contributed to the study.