A year of wearable data from 462 adults, checked against 376,758 Oura users, suggests how far someone falls below their personal norm predicts more than a fixed cutoff does.

Sleep research has long used a fixed threshold. Under six hours is short sleep, and the epidemiology linking short sleep to cardiovascular and metabolic risk mostly rests on that line. The line is a population average, though, and people differ substantially in how much sleep they need.

A study in Sleep Medicine from the National University of Singapore proposes replacing it with a threshold calculated per person. The result is that the personalised version predicts more outcomes more strongly than the fixed one — and the fixed one is not far wrong on average.

The first author is Chun Siong Soon, with Professor Michael Chee, Director of the Centre for Sleep and Cognition at the Yong Loo Lin School of Medicine, as senior author.

How the threshold is built

The study followed 462 working adults who wore Oura Rings and completed daily smartphone assessments for up to a year. From that, the team derived what they call personalised short sleep, or pSS: a night counts as short if it falls more than an hour below that individual’s own 75th percentile of sleep duration.

Using a high percentile as the reference rather than the mean is a deliberate choice, and a sensible one. A mean is dragged down by the short nights you are trying to identify; the 75th percentile approximates what a person sleeps when circumstances allow, which is closer to a personal requirement than an average that includes every late night.

“Population thresholds remain useful, but once we have months of data from an individual, we can ask something they cannot tell us: when is this person sleeping substantially less than is usual for them?” Chee said.

The two definitions turn out to flag similar amounts of sleep. About 35 per cent of nights were short under the conventional under-six-hours rule, against about 37 per cent under the personalised approach, and the median personalised threshold was about 6.1 hours — close enough to six that the population figure looks well chosen.

What the personalised version predicts better

The difference is in what the two definitions are associated with. Personalised short sleep showed larger effect estimates than the fixed definition across a wide range of measures: alertness, sleep satisfaction, stress, motivation, sadness, sleeping heart rate, heart-rate variability, sedentary behaviour and physical activity. The effects grew with consecutive short nights.

Personalised short nights were also more strongly associated with body roundness index, systolic blood pressure, pulse-wave velocity and arterial stiffness.

The logic of why a personal threshold should do better is straightforward. Someone whose natural requirement is nine hours and who sleeps six and a half is substantially deprived and invisible to a six-hour rule; someone who functions on six is counted as deprived every night by the same rule. Measuring the shortfall relative to each person’s own norm removes both errors at once.

The recovery arithmetic

The most practically interesting findings concern what happens afterwards, and they argue against a common assumption.

For 86.4 per cent of participants, the median run of personalised short sleep was a single night. After one such night, participants slept about 50 minutes more the following weekday night — and each additional consecutive short night added under 10 minutes of further recovery.

Weekend recovery reached up to about 1.25 hours but plateaued after several nights.

That pattern says the body does not run a full ledger. A single shortfall prompts a substantial rebound; a sustained one does not prompt a proportionally larger rebound, and the weekend cannot settle an accumulated debt because recovery sleep saturates. Which makes the frequency of shortfalls, rather than any single bad night, the thing that accumulates.

“An occasional short night is part of normal life,” Soon said. “What appears more informative is whether substantial shortfalls keep recurring, and particularly whether they accumulate across consecutive nights.”

“How much you sleep matters, but our findings suggest that how often you fall substantially short of your own usual sleep matters too,” Chee said.

Scale, and what it buys

To test whether the framework travels, the team applied it to anonymised data from 376,758 Oura users across Australia, Finland, Germany, Japan, Singapore, the UK and the US.

The national differences came out as the sleep literature would predict — Australia with earlier, longer sleep and fewer short nights, Japan with later, shorter sleep and more frequent shortfall — while the overall pattern held in all seven countries.

“Wearables give us an opportunity sleep science has never previously had at scale,” Chee said.

That is true and it carries a specific cost. Oura users are self-selected: people who buy a sleep-tracking ring and keep wearing it are not a random sample of any population, and they skew toward those with the income and the interest to monitor their own sleep. A dataset of 376,758 such people has enormous statistical power and unknown representativeness, which makes it good for testing whether a method generalises and poor for estimating how much short sleep exists anywhere.

What a wearable measures

Two further limits belong with the result.

Consumer rings estimate sleep from movement, heart rate and temperature rather than measuring it. They are reasonably good at total sleep duration and considerably less reliable at sleep staging, and the central variable here is duration — which is the measure they handle best, though an estimate rather than polysomnography.

The associations are also observational throughout. A night of short sleep followed by lower alertness and higher stress the next day is consistent with the sleep causing both, and equally consistent with stress having caused the short night; the cardiovascular measures are cross-sectional correlations in a cohort, not outcomes of an intervention. Nothing here shows that raising someone’s sleep to their personal threshold would change their blood pressure.

The release does not state the study’s funding sources or any conflicts of interest — which is a gap worth noting in work whose central dataset belongs to a commercial device company, since whether that company funded or participated in the analysis is something a reader should be told. The paper was published on 30 September 2026.

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Notes

1. Soon, C.S., Chee, M., et al. “Personalized definition of short sleep using long-term wearable sleep distributions.” Sleep Medicine, 2026. DOI: 10.1016/j.sleep.2026.109294.
2. National University of Singapore, Yong Loo Lin School of Medicine. “”How short is too short?” Long-term wearable data point to a more personal way of defining insufficient sleep.” EurekAlert!, 5 October 2026. https://www.eurekalert.org/news-releases/1146599.

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