HERSHEY, Pa. — When people think about ways to improve cardiovascular health, diet and exercise are often at the top of the list. But long-term health, especially in adolescents, might start with something more fundamental: sleep.
A new study, led by a team from Penn State College of Medicine, found that when teenagers go to sleep and when they wake up may be the driving force behind what teenagers eat and how much they move. Teens who went to sleep later and woke up later were more likely to consume more calories, snack more and be less physically active — especially when in school compared to on break from school. The findings suggest that sleep could be a lever for protecting heart health, the researchers said.
The study, which was published in the journal Sleep Health , examined how different aspects of sleep — beyond just hours slept — are associated with diet intake and composition, exercise and sedentary behavior in adolescents.
"Sleep is a potential risk factor for cardiometabolic health, even in teens," said Julio Fernandez-Mendoza , the Edward O. Bixler professor of psychiatry and behavioral health at Penn State College of Medicine and senior author of the study. "Sleep timing — when teens go to bed and wake up — had the biggest influence on sedentary and eating behavior in teens. It's something parents need to pay attention to — and protect — during critical developmental years like adolescence."
The body's internal clock regulates the sleep-wake cycle over 24-hours each day. But it governs other key bodily processes and behaviors too, like metabolism and physical activity. For example, going to bed and waking up late doesn't just impact a person's sleep schedule, it also influences their sense of hunger, craving for certain types of food and their desire to move or rest.
"We have the tendency to separate sleep, diet and physical activity as three distinct things, but we can't isolate them from one another. We have to think about them together," Fernandez-Mendoza said.
Most adolescents don't get the eight-to-ten hours of sleep recommended by the National Sleep Foundation . Teens' internal clocks naturally shift later into the evening during adolescence, which explains why teens may tend to stay up later and sleep in. However, typical school schedules are at odds with teens' natural drive for sleep. With early school schedules, they need to wake earlier than what their body clock naturally prefers.
Prior studies have found that when adolescents don't get enough sleep, they tend to be less physically active and to eat poorly, both of which can raise the risk of cardiovascular and metabolic disease. But these studies evaluated sleep based on a single metric, like sleep duration, and self-reported measures, the researchers said. This potentially limits the understanding of the extent of the relationship between sleep, diet and physical activity.
"Sleep is more than just how long a person sleeps but there aren't many studies that look at this issue from a holistic perspective beyond how much sleep teens get," said Pura Ballester-Navarro, professor at Universidad Católica San Antonio de Murcia in Spain and first author of the study.
In this study, the researchers wanted to determine if specific aspects of sleep — such as sleep duration, timing, regularity and quality — measured by distinct methods related differently to adolescents' eating habits and physical activity behaviors, and whether being in school or out of school changes how strong those relationships were.
The study included 373 participants from the Penn State Child Cohort, a longitudinal, population-based study established in 2000. Participants — a mix of males and females — were between the ages of 12 and 23, with an average age of 16.4 years. One set of participants was evaluated while they were in school, and another set was evaluated while they were on break from school.
The researchers monitored multiple aspects of sleep, including bedtime, wake time, total sleep time, sleep midpoint and its irregularity, sleep efficiency and time in bed. They collected data using a combination of objective and subjective methods including wrist-worn wearables, self-report surveys and in-lab sleep studies. They also tracked food intake, snack intake and physical activity.
The team found that adolescents who were "night owls," generally going to bed after midnight and rising after 8 a.m., consistently ate more calories, particularly carbohydrates, and were more sedentary. They also tended to snack more, especially later in the day and at night. Because they woke up later, they often skipped breakfast. Instead, they ate lunch, dinner and a late-evening snack, which tended to be less healthy than a typical breakfast. Highly variable sleep duration — when teens alternate nights of shorter and longer amount of sleep — was also associated with less healthy behavior, particularly less physical activity.
The influence of sleep timing and variability on diet and physical activity was two times stronger when school was in session. When teens are forced to sync up with an external schedule and fight their natural biological rhythms, it appeared to have a cascading effect on eating and sedentary behaviors. These relationships seemed to weaken during school breaks, when teens have more flexibility with their schedule. However, increased snacking behavior was observed when kids weren't in school.
"When the timing of teens' eating and snacking is out of sync with their normal biological clock, it further dysregulates their sleep," Fernandez-Mendoza said.
When trying to encourage healthy eating and physical activity, targeting the regularity and timing of adolescents' sleep could be a key strategy, the researchers said. For example, parents and caregivers can focus on earlier bedtimes, longer sleep duration and consistent sleep schedules during the school year while reducing late-night snacking and sedentary behavior when kids are out of school.
"A consistent sleep routine is a powerful tool," said Ballester-Navarro.
Other Penn State authors on the paper include Kristina Lenker, assistant professor of psychiatry and behavioral health; Susan Calhoun, associate professor of psychiatry and behavioral health; Jason Liao, professor of public health sciences; Duanping Liao, professor emeritus of public health sciences; Edward O. Bixler, professor emeritus of psychiatry and behavioral health; and Alexandros N. Vgontzas, the Anthony Kales, MD, University Chair in Sleep Disorders Medicine and professor of psychiatry and behavioral health.
Natasha Morales-Ghinaglia, assistant professor of anatomy and embryology at Geisinger Commonwealth School of Medicine, and Casandra Nyhuis, postdoctoral fellow at Johns Hopkins Bloomberg School of Public Health, both of whom earned their doctorates from Penn State, also contributed to the paper.
Funding from the National Institutes of Health (Fernandez-Mendoza) and the Fundación Seneca-Science and Technology Agency of Murcia (Ballester-Navarro) funded this work.
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