Remember dragging yourself out of bed before dawn to get to your high school classes on time? Remember how much easier it seemed in grade school? Yeah, you weren't just getting lazy, a new study shows.
The research, out today in the journal PLoS One, is further confirmation for what sleep researchers had been thinking for a while: As they get older, kids' and teens' circadian rhythms shift, meaning they really should go to bed later at night and wake up later in the morning. The new study lends support to the American Academy of Pediatrics' recent statement that middle and high schools shouldn't start earlier than 8:30 a.m.—doing otherwise, the pediatricians' group argues, could threaten kids' health and academic performance.
In their study, Stephanie Crowley and researchers from five other institutions followed 38 kids aged nine or 10 initially and 56 teens aged 15 or 16 initially, all from Providence, Rhode Island, for about two and a half years. About every six months, those 94 participants underwent a week-long sleep assessment, during which they kept daily sleep diaries and wore activity monitors on their wrists. The resulting data gave the research team an idea of their subjects' sleep patterns, but to investigate what sleep their bodies actually wanted, as opposed to what they got, the team had each participant come into the lab to measure something called dim light melatonin onset. Basically, that's the time when the body starts producing more melatonin in preparation for sleep.
As both the younger and older cohorts aged, the team found, they went to bed later and later, and on weekends they woke up at correspondingly later times, typically around 8 or 8:30 a.m.. On weekdays, kids under 18 all got up before 7 a.m.—suggesting schools set the de facto wake up time for most adolescents—while those 18 or older woke up closer to 8 a.m., in line with their weekend habits. Regardless of when they got up, the study participants' melatonin rhythms shifted back by one or two hours as they aged. That suggests that regardless of school policy, adolescents really ought to go to bed later.
"The consistent early weekday sleep offset [waking] times across 9 to 17 years ... indicates that the school schedule may suppress a biologically-driven behavior to sleep later," a result bolstered by the facts that weekend waking times grew later over time and that the difference between weekday and weekend waking times declined only after age 17, near or after the end of high school, the authors write. The conflict between school start times and biology could pose health risks as an apparently natural desire to stay in bed gets rudely awakened by the morning bell.