Timing and regularity of sleep may be key for adolescent heart health

HERSHEY, Pa. — There is a truth universally acknowledged that kids need sleep, a lot of sleep. Yet, most adolescents fall short of the eight-to-ten hours of shut-eye recommended by the National Sleep Foundation. However, total amount of sleep isn’t the only thing that matters for adolescent health and development. Late nights and erratic sleep schedules can impact young people’s heart health in later life, according to a team led by researchers from Penn State College of Medicine.

In a new study, the team found two distinct patterns that could contribute to increased cardiac risk. They evaluated two separate sets of adolescents during different times of the year. During the school year, teens with irregular sleep schedules on weekdays had lower heart rate variability (HRV), a key marker of cardiovascular function. On non-school days, like vacations, teens who go to sleep later and sleep in also had lower HRV. Both sleep patterns are signs that an adolescent’s internal body clock is out-of-sync with their sleep-wake cycle and academic, social or work schedules.

The findings, recently published in the journal SLEEP, were independent of factors such as demographics and total amount of sleep. While the researchers said they don’t know if these changes are permanent, blunted HRV is predictive of increased cardiovascular disease risk. The findings suggest that maintaining a regular sleep-wake cycle may protect adolescents from future adverse cardiac outcomes.

“A significant sleep delay or irregular schedule during adolescence can become a significant issue,” said Julio Fernandez-Mendoza, Edward O. Bixler professor of psychiatry and behavioral health at Penn State College of Medicine and senior author of the study. “We need to identify those whose sleep schedule is not well aligned with the nighttime period, monitor them and target them early in life to prevent the development of cardiometabolic disease in the future.”

Everyone has an internal clock, or circadian rhythm, that dictates their sleep-wake cycle. During adolescence and puberty, the internal clock matures, shifting teens to become more “evening types,” Fernandez-Mendoza said. They tend to stay up at night because they don’t feel as tired. In turn, they naturally may want to wake up later, making them less alert in the morning. In other words, a teen’s late nights aren’t just a matter of behavior, wanting to stay up to play video games or scroll on their phone — there’s a biological component, too.

The problem arises when there’s a mismatch between an adolescent’s circadian rhythm and their school or work schedule, called circadian misalignment. For example, while teens’ internal clocks have shifted later into the evening, they still need to wake up for school earlier than what their natural body clock prefers.

“The typical school schedule, for many teens, is at odds with their normal maturation. These kids are misaligned and are sleep deprived and that’s when we’ll start to see adverse outcomes,” Fernandez-Mendoza said. He explained that prior studies in the field have investigated the link between circadian misalignment and academic performance as well as mood and emotional regulation. For this study, his team focused on the connection between sleep timing and cardiovascular health.

The study included 360 participants from the Penn State Child Cohort, a random, population-based study established in 2000. Participants — a mix of males and females representing multiple racial or ethnic minorities — were between the ages of 12 and 23 with an average age of 16.3.

The researchers monitored HRV, the variation in time between each heartbeat. They explained that HRV is a measure of cardiac autonomic modulation, which reflects the balance between the sympathetic and parasympathetic nervous systems or the fight-or-flight and rest-and-digest systems, respectively.

A healthy heart modulates between these two systems, reacting to the body’s needs. For example, when stress or excitement activates the sympathetic nervous system, the heart responds by speeding up. That response is then countered by an increase in parasympathetic activity, slowing the heart down. He explained that a high HRV is a sign of the heart’s ability to adapt well to stress. A low or blunted HRV, on the other hand, indicates that the heart is less resilient. In adults, blunted HRV is linked to future heart disease.

“It’s a good sub-clinical marker of cardiovascular disease risk, especially in this age group where these kids may not have developed heart disease yet,” Fernandez-Mendoza said.

One set of participants was evaluated while in school and another set was evaluated while on break. Participants were monitored in the sleep lab overnight where the research team observed their sleep and HRV. When they left the lab, they continued to be monitored at-home. During the week following the in-lab visit, participants wore a sensor to track their sleep and kept a sleep log. They also wore a Holter monitor, a small device that records heart rhythm, to monitor HRV during the day and night for 24 hours.

The main metric evaluated by the team was sleep midpoint, or the time halfway between falling asleep and waking up. For example, if a teen goes to bed at 11:30 p.m. and wakes up at 6:30 a.m., their sleep midpoint would be 3 a.m.  

The researchers found two distinct patterns of circadian misalignment that were associated with blunted HRV in adolescents, which suggests an overactive sympathetic response or an inability of the parasympathetic system to downregulate the sympathetic response.

First, adolescents who had a later sleep midpoint on the weekends while on break from school, compared to the average amongst the study’s participants, showed a significant decrease in nighttime HRV. The researchers explained since teens don’t have to adhere to a strict schedule on these days, they’re more like to express their natural sleep-wake cycle by delaying their sleep and waking up later in the morning. Second, adolescents with inconsistent sleep schedules during the school year, meaning that there was greater fluctuation in their sleep midpoint night-to-night compared to the average, had lower daytime and nighttime HRV.

In both cases, the data suggests that a shift of approximately one hour compared to the average was associated with negative changes in HRV. However, Fernandez-Mendoza explained that there can be a lot of variability, particularly given the inconsistency of adolescent sleep schedules, and noted that other studies have found that a delay of two and a half hours has been associated with adverse outcomes.

“The effects of delayed and irregular sleep timing are highly dependent on the conditions under which adolescents are evaluated — whether during the school year or on-break — because academic and social responsibilities can conflict with their natural circadian rhythms, negatively impacting cardiac health,” said first author Natasha Morales-Ghinaglia, assistant professor of anatomy and embryology at Geisinger Commonwealth School of Medicine, who earned her doctorate from Penn State.

In other words, whether adolescents are evaluated while in-school or on-break will play a role in determining which measure of circadian misalignment is most strongly associated with cardiac autonomic modulation.

“Our findings highlight the importance of assessing adolescents' sleep patterns and timing during medical visits to identify those with irregular sleep-wake schedules and may be at risk for blunted HRV, which can contribute to cardiovascular disease later in life,” Morales-Ghinaglia said. “There’s so much that we can do now to help improve sleep and protect their health.”

Other Penn State College of Medicine authors on the paper include Susan Calhoun, associate professor of psychiatry and behavioral health; Alexandros Vgontzas, professor and Anthony Kales, MD, University Chair in Sleep Disorders Medicine; Jiangang Liao, professor of public health sciences; Duanping Liao, professor emeritus; Fan He, instructor, public health sciences; and Edward Bixler, professor emeritus.

Funding from the National Heart, Lung, and Blood Institute, the National Institute of Mental Health and the National Center for Advancing Translational Sciences of the National Institutes of Health along with the American Heart Association supported this work.