Almost everything we are told about sleep comes down to one instruction: get your hours. Seven to nine, the figure on every wellness checklist, repeated until it feels like the whole of the matter. Sleep gets treated as a quantity to be filled.
There is a second question folded inside sleep that the hours never think to ask. Not how long you slept, but when. What time your body crossed from waking into sleep last night, and the night before, and whether those crossings happened near each other. The hours measure the size of the night. They say nothing about its place.
That second measure has a name now, and research behind it. You can reach your 7 to 9 hours and still keep a schedule your body reads as disorder. Normal counts the hours. The body, quietly and without being asked, also keeps time.
The half of sleep we never counted
For most of the time we have studied sleep, the question seemed obvious: how much. Hours were easy to count and easy to compare, so duration became the number that stood in for sleep itself. How consistently a person slept from night to night, the rhythm rather than the length, went largely unmeasured. It was the half of sleep nobody had a ruler for.
That changed in 2017, when Andrew Phillips and colleagues found a way to put a number on consistency. They called it the Sleep Regularity Index. Take any two moments exactly 24 hours apart and ask whether the body was in the same state, asleep or awake, at both. Do that across a week, average the answers, and you have a single score. Unlike older measures, it makes no assumption that sleep arrives in one tidy block, so naps and broken nights are counted honestly rather than ignored.
Sleep regularity is the day-to-day consistency of when you sleep and wake, distinct from sleep duration, which is how long. It is captured by the Sleep Regularity Index, introduced in 2017 and scored from 0 to 100: a score of 100 means falling asleep and waking at the same times every day, while 0 means entirely random timing.
The clock beneath the hours
To see why timing matters, it helps to look at what sleep keeps time with. The body runs on a roughly 24-hour internal clock, the circadian rhythm, and it is not a metaphor. A master clock in the brain, a cluster of cells in the hypothalamus called the suprachiasmatic nucleus, sets the tempo for nearly every system beneath it, from hormone release to body temperature to digestion.
That clock does not keep perfect time on its own. It is set, day after day, mostly by light. Morning light reaching the eye anchors the clock to the world outside; darkness cues the pineal gland to release melatonin, the body's own signal that night has come. Melatonin here is physiology, a hormone the body makes for itself, in the same family of internal signals as cortisol.
The trouble with an irregular schedule is that it keeps moving the target. When sleep timing lurches by hours from one day to the next, the clock is forced to re-align to a world that will not hold still, a state researchers call circadian misalignment. The body never quite finishes setting itself before it must start over.
Morning light is the strongest signal that resets the body's internal clock each day.
What the big study found
The most striking evidence for timing arrived in 2024, in the journal Sleep. Researchers led by Daniel Windred calculated regularity scores from more than ten million hours of movement data, gathered by wrist-worn sensors on roughly sixty thousand UK Biobank adults, with an average age near sixty-three. Then they watched what followed, up to about 7.8 years.
The pattern was consistent. Compared with the least regular fifth of sleepers, those in the more regular four-fifths were associated with a twenty to forty-eight percent lower risk of death from any cause, a sixteen to thirty-nine percent lower risk of death from cancer, and a twenty-two to fifty-seven percent lower risk of death from cardiometabolic causes, after accounting for age, sex, ethnicity, lifestyle, and health. A second large analysis of the same group found a similar relationship.
The finding that reframes everything is this: in their data, regularity predicted who lived longer more strongly than duration did. This is an observation across a population, not a verdict on any one person, and it cannot say steady sleep prevents disease. The authors noted that for many people, sleeping more regularly may be a simpler, more reachable goal than sleeping more.
Enough sleep, badly timed
Here is the part that surprises people: you can sleep plenty and still sleep irregularly. Across the research, from college students in the 2017 work to older adults in later studies, regularity has turned out to be largely independent of duration. A person can average a comfortable eight hours and still drift their timing widely enough to score as irregular, and less regular sleep has been linked, in observational data, with later timing, more daytime sleepiness, less daylight exposure, lower mood, and higher cardiometabolic risk.
Most of us know the everyday version of this, even if we never named it. Stay up late Friday and Saturday, sleep in to catch up, then face a weekday alarm on Monday, and you have given yourself jet lag without leaving your time zone. Chronobiologists named the effect in 2006: social jetlag, the gap between when you sleep on work days and on free days. An estimated sixty-nine to seventy percent of adults in industrialized countries carry at least an hour of it, part of why Monday can feel like jet lag. Beyond the hours lost, it has been associated with higher body weight and metabolic strain, though here too the studies show association, not proven cause.
Irregular timing behaves like a self-made jet lag the body never fully re-aligns from.
What regular actually looks like
So what does regular actually ask of you? Less than people fear. The most useful anchor, sleep scientists agree, is a consistent wake time held about seven days a week, because morning is when light most strongly sets the clock. Step into daylight within 30 to 60 minutes of waking, since outdoor light is far brighter than indoor and a screen is a poor substitute. If you need to shift, move gently, about 30 minutes a day, because the circadian system is slow to turn. Most people settle into a new rhythm within about 3 days, and keeping weekend times within 30 to 60 minutes of weekdays holds most of the gain.
In 2023 the National Sleep Foundation issued a consensus statement on sleep regularity, chaired by Harvard's Charles Czeisler, after reviewing more than sixty studies. It concluded that consistent bed and wake times matter for cardiovascular, metabolic, cognitive, and mental health, and that roughly seventy percent of US adults do not get sufficient sleep regularly.
The panel was careful about what it did not say: not identical times at any cost. When sleep debt has piled up across the work week, it concluded the priority is to minimize that deficiency, and that 1 to 2 hours of catch-up sleep, or a nap, on a free day can help. Regularity is the sensible default, not a rule to defend against the sleep you genuinely need.
How we read it
A measure this new deserves humility, and the science offers it. The Sleep Regularity Index has existed only since 2017, and in 2025 a group working on reporting standards found that different software can produce noticeably different scores from the same nights, so the precise number matters less than the direction it points. Much of the evidence is observational, and some of it leans on what people report about their own sleep. And some lives sit outside the tidy version of this advice: shift workers, new parents in the thick of broken nights, and anyone with insomnia or a diagnosed sleep disorder are a different conversation, better had with a physician than a checklist.
What holds steady through it all is the direction. More consistent is better, and it is something most people can reach without sleeping a single hour more. At the practice, we read a signal like this the way the body writes it: a pattern across weeks and seasons, with a physician, not a single night judged alone. One short night is noise. The shape of a hundred is the story.
So if you change one thing, let it be the hour you wake, kept close enough each day that your body stops having to guess, then step into the morning light. The hours will still matter, and they always did. But the body has been keeping a second kind of time all along, beneath the count, waiting for the rhythm to hold.
Common questions
What is sleep regularity?
Sleep regularity is the day-to-day consistency of when you fall asleep and wake, as opposed to sleep duration, which is how many hours you sleep. A person with high sleep regularity goes to bed and rises at close to the same times each day, including weekends. It is most often measured by the Sleep Regularity Index, a score from 0 to 100 introduced in 2017.
Is sleep regularity more important than sleep duration?
Both matter, but in a large 2024 study of roughly sixty thousand adults, sleep regularity predicted who lived longer more strongly than sleep duration did. The two are largely independent, which means you can sleep enough hours and still keep an irregular schedule. These are observational findings, so they describe association rather than cause, but they suggest consistency deserves as much attention as the number of hours.
What is the Sleep Regularity Index (SRI)?
The Sleep Regularity Index, or SRI, is a measure introduced by Andrew Phillips and colleagues in 2017 that scores how consistent sleep-wake timing is from day to day. It is the probability of being in the same state, asleep or awake, at any two points 24 hours apart, averaged across about a week and scored from 0 to 100. A score of 100 means identical timing every day, while 0 means entirely random timing.
What is social jetlag?
Social jetlag, a term coined in 2006, is the gap between your sleep timing on work days and on free days, such as staying up late and sleeping in on weekends. It creates a misalignment between your internal clock and your schedule similar to the jet lag of crossing time zones, but without any travel. An estimated sixty-nine to seventy percent of adults in industrialized countries experience at least an hour of it.
How do I make my sleep more regular?
The strongest single step is anchoring a consistent wake time you can keep about seven days a week, since morning is when light sets the body's clock. Get outdoor light within 30 to 60 minutes of waking, keep weekend times within 30 to 60 minutes of weekdays, and if you need to shift your schedule, move it gradually by about 30 minutes a day. People with insomnia, a sleep disorder, or shift work should speak with a physician.
