College students are convenient guinea pigs. Living on the campuses of leading research institutions and easily enticed by gift cards and other one-off perks, they’re the most scientifically scrutinized humans in history, and studies involving college students regularly catalyze novel ideas for further research.
One such idea, arising relatively recently, is the notion that sleeping consistently is more important than sleeping longer — for both restfulness and overall health.
Irregular sleepers
College students might be the perfect demographic for studying this concept, largely due to their erratic sleep schedules. A decade ago, Andrew J.K. Phillips and William M. Clerx, then sleep researchers at Harvard Medical School, recruited 63 full-time undergraduates at Harvard College for a study on sleep. Phillips and Clerx were curious whether sleep regularity, the day-to-day variability in sleep duration, onset time, wake time, or quality, could be linked to academic performance.
Participants tracked their sleep via online diaries for a month. A subset also wore specialized watches that measured light exposure. They also donated saliva to gauge hormone levels.
This was a seismic finding, with ramifications that have not yet percolated to the mainstream.
With this data, Phillips and Clerx devised the sleep regularity index (SRI). SRI ranges from 0 to 100 and gives the probability that an individual falls in the same sleep or wake state at any two time points 24 hours apart. For example, a person who went to bed at 10 p.m. and woke up at 6 a.m. two days in a row would score a perfect 100. As sleep-wake times deviate from one day to the next, the score decreases. A person who scores 0 essentially sleeps at random.
Phillips and Clerx averaged each participant’s SRI over the study’s duration, then classified individuals scoring in the top quintile as regular sleepers and individuals scoring in the bottom quintile as irregular sleepers. Interestingly, both groups slept about the same amount each day, a little over seven hours. Regular sleepers, however, tended to go to bed “earlier” (1 a.m. v.s. 3 a.m.), took fewer naps, and spent more time asleep at night.
The difference in sleep styles was linked to a significant disparity in academic performance: Regular sleepers averaged a 3.72 GPA, while irregular sleepers averaged a 3.42.
Not just grades
Interesting, sure. But maybe the regular sleepers simply studied more and partied less?
Phillips and Clerx’s finding could’ve been offhandedly waived away, but it wasn’t. Instead, other researchers utilized SRI to explore the effects of sleep regularity in the broader population. What they found was eye-opening.
While the scientific literature on “sleep regularity” was essentially nonexistent before 2020, there have now been more than 300 studies published in the past six years. They’ve shown that irregular sleep is linked to epigenetic aging, depression, lower quality of life, cancer, obesity, hypertension, dementia, cardiovascular disease, and death. Suffice to say, sleep regularity isn’t just about grades anymore.
Phillips, now a professor at Flinders University and the Director of Circadian Health at Flinders Health and Medical Research Institute, was senior author on a major study published in 2023 that revealed a link between irregular sleep and mortality. Relative to those in the lowest quintile of SRI, sleepers in the highest quintile enjoyed a 30% lower risk of mortality from all causes. Even more striking, Phillips and his co-authors found that sleep regularity was a far stronger predictor of death than the conventional measure of good sleep: duration. In other words, the study showed that when one slept was more impactful on health than how long.
Sleep regularity was ignored, but no longer
This was a seismic finding, with ramifications that have not yet percolated to the mainstream. Nearly 20,000 studies focusing on “sleep duration” have been published since 2000, compared to just a few hundred on “sleep regularity.” Popular media is full of tips on how to sleep more and nap better. The American Academy of Sleep Medicine (AASM) and Sleep Research Society (SRS) offer consensus statements on how long adults and children should sleep, but offer considerably less information about when and how consistently they should sleep.
Dr. Marie-Pierre St-Onge is the founding director of the Center of Excellence for Sleep and Circadian Research at Columbia University. In an email interview with Big Think, she explained why researchers ignored sleep regularity for so long relative to sleep duration.
“Most population-based studies were not collecting daily sleep data that included bedtime, wake time, and sleep duration,” St-Onge said. “They were asking participants, ‘On average, how much sleep do you get at night?’”
In part, this was simply because it was a tall task to ask thousands of participants in large health surveys to precisely track their sleep day in and day out. But this became much easier as inexpensive wearable devices that monitor sleep grew more commonplace.
“Ancillary studies to ongoing studies, specifically focused on sleep, started collecting sleep data from devices over multiple days. That allowed for a more in-depth evaluation of sleep behaviors,” St-Onge said.
With this data, we’re not only illuminating sleep regularity’s effects on overall health, but also the physiological reasons behind why it’s so vital.
Circadian rhythms
The first clues to explaining sleep regularity’s health effects came from Phillips and Clerx’s initial study on college students. The light monitors that some students wore, as well as their melatonin levels, indicated that irregular sleepers were experiencing misalignment between their circadian systems and their sleep-wake cycles. In other words, they were constantly jet-lagged. Compared to students who slept regularly, it was like the irregular sleepers had traveled two to three time zones westward.
“We know that when people have irregular sleep patterns, the timing information their body receives from the environment is less consistent,” Phillips told Big Think. “This can potentially affect a whole host of functions, including metabolism, mood, cardiovascular function and immune function.”
Over the past few decades, scientists have learned the value of aligning one’s circadian rhythm, the body’s natural 24-hour internal clock, with light and dark cycles.
“In humans, these processes coordinate daily cycles of metabolism, immune functions, cardiovascular activity, and hormonal secretion, as well as behaviors and social activities,” Dr. Tianyi Huang, head of the Sleep Health and Aging-Related Phenotypes (SHARP) Unit at the National Institutes of Health, wrote in a review article on sleep regularity published last year.
St-Onge shed more light on how sleeping consistently aligns the body’s clock and boosts health.
“When we have stable sleep schedules, we tend to also have more stable eating patterns and provide nutrients in alignment with our body’s optimal times to process and assimilate those nutrients.”
A “hotbed” for circadian disruption
Some people are night owls, and some are early birds. Some people are naturally long sleepers and others short sleepers. But interestingly, a genome-wide association study for sleep traits published in 2019 reported a heritability estimate of just 2.8% for sleep regularity.
This suggests that sleeping consistently is rarely determined by genes. In other words, sleeping irregularly is a choice, or at least a behavior we slip into based on our habits, surroundings, or obligations.
Between ubiquitous artificial light, inexpensive junk food, distraction from screens, the preponderance of gig work, and long working hours, the “modern environment and lifestyle create a hotbed for circadian disruption and poor sleep health,” Huang wrote.
In his 2025 review, he cited research suggesting that 43% of Americans suffer from irregular sleep. Younger age, nonwhite race and ethnicity, living alone, depressive symptoms, and stress were consistently associated with sleep irregularity.
But again, almost nobody is predestined to sleep irregularly. We can take steps to sleep more consistently.
How to sleep regularly
In his 2023 study, Phillips offered an attainable goal.
“In our study, people with the top 20% of SRI scores went to sleep and awoke within approximately 1-hour windows on the majority of their study days,” he and his colleagues wrote. “Aiming to fall asleep and wake up within 1-hour windows each day may therefore be a feasible strategy.”
To hit that target, Phillips recommended paying attention to one’s light exposure.
“Focus on the most important circadian time cue, which is light,” he told Big Think. “Help your body to maintain regular and robust rhythms by having consistent exposure to natural light during the daytime, and using dimmer, warmer lighting in the hours leading up to bedtime.
St-Onge also offered some advice.
“The easiest way to ensure that we get adequate sleep, as much as possible, with regular schedules, is to determine the necessary wake time based on daily obligations,” she told Big Think. “Then, counting back from that, determine bedtime. Ideally, you should be able to go to bed and wake up around those times daily.”
Such a regimen doesn’t have to be extremely rigid, and thus potentially stressful, she said. Simply striving for consistency puts you on a path to actually sleeping consistently.
“Over time, the body habituates to a stable schedule, and it makes it easier to fall asleep and wake up naturally around those times.”
St-Onge has conducted numerous research studies exploring the interplay between nutrition and sleep, and even wrote a popular book putting that science into practice.
“Not eating too close to bedtime is helpful, and making sure to eat a diet that is higher in fiber (from fruits, vegetables, nuts, whole grains) and lower in saturated fat (less animal protein and solid fats),” she summarized, before providing an even simpler take-home message for promoting regular sleep.
“Keep eating and physical activity behaviors during daylight hours and consume fewer calories later in the day.”
For all the jet-lagged, irregular-sleeping college students out there, that means less late-night pizza and fewer early-morning dance parties. (Sorry!)
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