Some People Sleep Under 5 Hours and Show No Warning Signs, Study Finds

Sleeping under five hours a night is supposed to take a toll. Yet some people in a new study did exactly that, night after night, and showed no clear warning signs in their health. Others who logged a perfectly healthy-looking average quietly carried higher stress, worse moods, and elevated blood pressure. What separated the two groups wasn’t how much they slept. It was how steady their sleep stayed from one night to the next. For years, sleep researchers have used six hours as the common cutoff for calling a night’s sleep “too short,” but a new study from Singapore suggests that this one-size-fits-all line misses what’s actually normal for a given person.

Researchers at the National University of Singapore followed 462 healthy working adults for up to a year, using sleep-tracking rings and daily check-in surveys, and reported their findings in the journal Sleep Medicine. Instead of measuring everyone against the same six-hour cutoff, they also built a personalized sleep threshold for each person based on their usual sleep patterns. That personalized number showed much stronger links to the day-to-day effects of short sleep, from mood and focus to heart rate and blood pressure.

A single fixed cutoff would have missed both groups: naturally short, steady sleepers who were fine, and seemingly well-rested but erratic sleepers who weren’t.

How Scientists Measured a Personalized Sleep Threshold

Participants, with an average age of about 40, wore an Oura Ring sleep tracker and answered a short daily survey each evening for up to a year, reporting their mood, stress, motivation, and how well they’d slept the night before. The ring also tracked heart rate and heart rhythm patterns during sleep, along with daytime activity levels. At the start of the study, participants also underwent a baseline health check that measured blood pressure and arterial stiffness, a sign of long-term heart disease risk.

To compare approaches, researchers measured short sleep two different ways. The first was the standard rule: anything under six hours counts as short. The second was personal: researchers looked at each person’s own sleep history and set their short-sleep cutoff at one hour below the 75th percentile of their usual sleep, the upper quarter of their typical sleep range, representing the “higher-opportunity” nights with fewer schedule constraints. In plain terms, if someone usually logs around eight hours on their better nights, dropping below seven would count as short sleep for them specifically, even though seven hours looks perfectly fine by general standards.

Oddly, both methods flagged almost the same share of nights as short, about a third of all nights tracked. But that similarity was misleading. When researchers looked at which nights and people were actually flagged, the two approaches disagreed sharply. The personalized method caught trouble in people who were technically sleeping “enough” by general standards but were still falling short of their own needs. It also correctly cleared people whose short sleep was simply their normal, steady pattern.

That personalized approach showed stronger links to ten different day-to-day measures, including alertness, stress, sleep satisfaction, motivation, sadness, heart rate and heart rhythm during sleep, and physical activity. It also connected more closely to longer-term health markers, including blood pressure, body shape linked to obesity risk, and artery stiffness. Researchers ran extra checks to rule out random chance, and the results held up.

What a Personalized Sleep Threshold Reveals About Risk

One especially telling finding involved how often short sleep happened in a row. Most people with short nights had them in single, isolated bursts rather than long streaks. About 76 percent of people using the standard definition, and roughly 86 percent using the personalized one, typically had a short-sleep streak of just one night. But when short nights stacked up back-to-back, mood, stress, and physical health markers worsened with each additional night. A single rough night seemed easier for the body to bounce back from. Repeated short nights were a different story.

Researchers also looked at recovery. After a short night during the week, people only gained back about 50 extra minutes of sleep the next night, not nearly enough to make up the lost time. On weekends, catch-up sleep was more generous, sometimes over an hour longer. But even weekend catch-up sleep leveled off after several nights of short sleep in a row. The body, it seems, can’t fully repay a sleep debt just by sleeping in.

To see whether this pattern held up outside Singapore, the team also examined anonymous data from Oura Ring users in six other countries, including Japan, Australia, the United States, and the United Kingdom, covering hundreds of thousands of people. Sleep habits varied widely by country, with places like Japan showing shorter average sleep and more frequent short-sleep streaks than Australia. Yet the basic pattern, mostly isolated short nights with rare long streaks, showed up consistently everywhere.

Rethinking the Six-Hour Sleep Rule

Six hours isn’t a bad number across the board. The trouble comes from treating it as a universal cutoff, which misses what’s actually happening in individual bodies. Someone who consistently sleeps fewer hours may show few warning signs, while someone who hits the “recommended” range but bounces wildly from night to night could still carry health risks a six-hour cutoff misses. Sleep trackers can now spot that second group: people who look fine on paper but aren’t. Public health advice built entirely around one fixed number risks reassuring the wrong people and needlessly alarming others. According to this research, the real warning sign isn’t a single rough night. It’s a pattern of repeatedly falling short of what’s normal for a person’s own body.

Disclaimer: This article summarizes an observational study of generally healthy adults. It can show associations between personalized sleep patterns and health or mood markers, but it cannot prove that short or irregular sleep directly causes those outcomes, especially since heart measures were taken only once at the study’s start. Sleep needs vary from person to person, and anyone considering changes to their sleep should speak with a healthcare professional.

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