“How did you sleep?” You might answer that question by weighing how many hours you slept or how often you woke up throughout the night.
But there is a third, often neglected, element of sleep to consider, experts say. It’s the consistency of your sleep schedule.
Sleep consistency refers to how well you maintain the same bedtime and wake-up time, give or take 30 minutes — and that includes weekends, said Jean-Philippe Chaput, a professor of medicine at the University of Ottawa.
Research suggests that most adults in the United States do not have a consistent sleep schedule. And that may be harming their health, Dr. Chaput said.
What the Research Suggests
Much of the science on the link between inconsistent sleep and poor health is based on observational studies, which can’t prove cause and effect. Their results are also often restricted by various limitations (including if the study was performed on a small number of people, or on people of only certain ages, ethnicities or occupations). It’s also difficult to accurately track people’s sleep patterns over months or years, and some studies define sleep consistency in different ways.
Despite these limitations, scientists have found some patterns. Those who tend to deviate most from a consistent sleep schedule seem to be at higher risk of certain health conditions like cardiovascular disease, obesity, mental health issues like depression and anxiety, and dementia.
In a 2020 study, researchers analyzed the sleep patterns of nearly 2,000 adults aged 45 to 84 in the United States. They concluded that those with the most irregular sleep schedules were more than twice as likely to develop cardiovascular disease than those with more regular sleep patterns.
In another study published in 2024, researchers analyzed sleep data from more than 88,000 adults in the United Kingdom and assigned “sleep regularity” scores to all of them. Those who scored lowest, meaning they had the most irregular sleep schedules, were about 50 percent more likely to develop dementia than those who scored in the middle of the range.
Scientists aren’t sure how frequent or how severe your sleep irregularity has to be to increase your health risk, said Soomi Lee, an associate professor of sleep and aging at Penn State. But the more you deviate from your typical sleep time — whether that’s within a 24-hour period or across weeks or months — the more the risks seem to increase, she said.
In a large review of studies published in 2023, a group of sleep scientists concluded that there was enough evidence to recommend maintaining a regular sleep schedule to help protect metabolic, mental and cardiovascular health.
Why Inconsistent Sleep May Put Health at Risk
Researchers are still untangling why inconsistent sleep patterns might negatively affect health, but their leading theory has to do with the body’s circadian rhythm, Dr. Lee said.
Your circadian rhythm makes up a roughly 24-hour internal clock that governs your sleep-wake cycle as well as the ebbs and flows of your hormones, metabolism, cardiovascular function, immune system, appetite and mood.
When you stray from your typical sleep schedule, the bodily functions that rely on those rhythms are thrown off, too. For example, staying up late or sleeping in may affect your hormone levels. Cortisol, which regulates stress, could be released at odd times or in more erratic ways. This can increase stress and inflammation throughout the body that, over time, may affect cardiovascular or metabolic health, Dr. Chaput said.
A misaligned circadian rhythm may also cause you to feel hungry outside of your regular mealtimes, said Dr. Andrew Varga, an associate professor of medicine at the Icahn School of Medicine at Mount Sinai in New York City. That could lead you to eat at unusual hours, such as late at night, possibly resulting in digestive issues or, in the long run, weight gain or obesity, he said.
How to Keep Sleep Consistent
With work, school, parenting demands and social obligations, it can be challenging to sleep consistently. But experts have some tips.
Setting an alarm to go off an hour before your bedtime every night can remind you that it’s time to start getting ready for sleep, Dr. Varga said. Doing something relaxing during that hour, such as reading or meditating, can help you wind down for bed.
It’s also important to expose yourself to sunlight every morning — ideally for 20 to 30 minutes at the same time every day, said Dr. Nishay Chitkara, the director of sleep medicine at NYC Health + Hospitals/Bellevue. While standing in front of a window can be beneficial, he said, it’s best to go outside to do this, even if it’s cloudy. A bright artificial indoor light, like a light therapy box, can help, too.
Light is the main cue that regulates your circadian rhythm. When it hits your eyes in the morning, your body begins its countdown to later that evening — when it releases hormones telling your body it’s time to go to bed.
You may not feel exhausted from inconsistent sleep in the same way you might after a night of tossing and turning, Dr. Lee added. But try your best to stick with a sleep routine regardless. The more consistent you are, she said, the better your health will be in the long run.
Caroline Hopkins Legaspi is a Times reporter focusing on nutrition and sleep.
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