Your biorhythm: why the same half hour of sleep carries a different weight

When the evening eats into the night

You meant to go to bed on time tonight. Then there was still the washing-up, a message that needed an answer and an episode that ended at the most gripping moment, and the evening quietly ran on into the night. The alarm still goes off at the usual time. You get through the morning on coffee, sit at your desk in the afternoon, sit again on the way home and tell yourself the weekend will make up for the lost sleep. On Saturday you stay up late, on Sunday you sleep in, and on Monday your body clock seems to live in a different time zone from your diary.

Most advice keeps the parts of that day separate: sleep more, sit less, move more. But every part comes out of the same 24 hours, and every minute you give to one is a minute you take from another. That pattern is your biorhythm: the shape your body’s day takes, from the moment you fall asleep through to the hours in which you sit, stand and move. This background article is about what happens when half an hour shifts within that rhythm, and why a new study suggests that the answer depends on how much you already sleep.

A large part of everyone’s day

The sources for this article contain no official Dutch count of sleep time, but the German Federal Statistical Office does keep one. Destatis reported in 2024 that people in Germany slept an average of 8 hours and 37 minutes a day in 2022.⁠² In the same 2024 publication, Destatis wrote that this was 8 minutes more than ten years earlier.⁠² Destatis also reported in 2024 that couples with children at home sleep an average of 19 minutes a day less than couples without children at home.⁠²

Those figures tell you two things. Sleep fills more than a third of an average day, so whatever you add to your evening usually comes out of the night. And you are not the only one making that trade. In households with children, the rest of life takes sleep minutes away every day on average. If everything else keeps squeezing your nights, you are one of many.

How much of your night is deep sleep is a separate question, covered in how much deep sleep you get per night.

What was already known about sleep, sitting and biorhythm

Why does the day keep shifting? Because the day cannot get any longer. Work, travel, family and screens all claim hours, and the morning usually starts at a fixed time, so the evening stretches and the night shrinks. You may notice it when you wake up without feeling rested; that is what why you wake up tired is about.

Research had already looked at this rhythm in large groups. The UK Biobank is a large British cohort of research participants. Among the participants whose sleep was measured with a small wearable device, a regular sleep-wake rhythm went together with a lower risk of dying, and regularity predicted that risk better than the number of hours slept.⁠³ A separate analysis of the same cohort found that irregular sleep-wake patterns are linked to a higher risk of dying.⁠⁴

Standing has a story of its own. In UK Biobank data collected with a small wearable device, the unfavourable links of time people spent standing still or sitting came mainly from the sitting. More standing in place of sitting did not go together with better heart and blood vessel health.⁠⁵ So standing is not simply the opposite of sitting.

Earlier research had also linked shifting time between sleep, sitting, standing and activity to the risk of dying early. What remained unclear was whether those links are the same for everyone, or whether they change with how long and how regularly someone sleeps.⁠¹ That is where the knowledge stopped.

New research: the whole day at once

On 7 October 2026, the American Journal of Epidemiology published a study aimed squarely at that gap in our knowledge.⁠¹ The study comes from the Mackenzie Wearables Research Hub at the Charles Perkins Centre and the School of Health Sciences at the University of Sydney. First author W. Bian and co-authors M.N. Ahmadi, N.A. Koemel and E. Stamatakis work there, and Ahmadi, Koemel and Stamatakis are also affiliated with the Turner Institute for Brain and Mental Health at Monash University in Melbourne. The other authors are R.K. Biswas of the School of Health Sciences at the University of Sydney; J.J. Mitchell, J.M. Blodgett and M. Hamer of UCL in London; D. Dumuid of the University of South Australia; S.F.M. Chastin of Glasgow Caledonian University and Universiteit Gent; and J.P. Chaput of the University of Ottawa and the Children’s Hospital of Eastern Ontario Research Institute. The article appeared in a scientific journal and is not a preprint.

Why does this matter to you? If you try to fit more movement into a full day, the time has to come from somewhere, and the night is the obvious place. Advice to move more rarely says what you should give up to do it. This study asks that question directly. It asks it separately for people who sleep enough and people who sleep little, and for people with regular and people with irregular sleep. The answer suggests that the same half hour can count differently, depending on the night it comes from.

Your day is a 24-hour budget

The one idea you really need to grasp here is the closed day. You cannot add time to anything without taking it away from something else. A study that only asks whether exercise is good for you compares people whose extra exercise time came from somewhere, and can leave open where that time came from. The researchers in this study took the opposite route. They looked at the day as five parts that always add up to 24 hours: sleep, sitting, standing, light activity and moderate to vigorous activity. All five were measured with a small wearable device rather than reported from memory.⁠¹ Light activity is gentle movement, such as pottering around the house. Moderate to vigorous activity is the kind of movement that makes you breathe faster, such as brisk walking.

The statistical method the researchers used is designed for parts of a whole. It does not ask what more sitting does on its own. It asks what goes together with shifting a fixed amount of time, here 30 minutes, from one part to another while the rest of the day stays as it is. The method then compares the risk of dying in the years that follow. It is a comparison between people whose days had a different shape. It is not an experiment in which anyone changed their own day.

Looking at the day as one whole is still unusual. Nicholas Koemel, one of the co-authors, described the problem in a University of Sydney press release about another 2026 study by the same research group:

Sleep, physical activity and diet are all factors known to be linked to a healthier life, but these factors are usually studied separately from one another.

Nicholas Koemel, lead researcher at the Mackenzie Wearables Hub at the Charles Perkins Centre and co-author of the new study, The University of Sydney⁠⁶

The study followed 68,995 adults from the UK Biobank. Over an average of 8.0 years, 2,668 of them died.⁠¹ The team then split the analysis by sleep: people who met the guidelines for sleep duration and people who slept short, and people with regular and people with irregular sleep.

Take an evening walk that comes out of a full night. In people who already met the guidelines for sleep duration, shifting 30 minutes from sleep to standing, to light activity or to moderate to vigorous activity went together with a slightly lower risk of dying from any cause during the follow-up period. The difference was largest for the vigorous form of activity.⁠¹ In this group, half an hour less in bed in exchange for movement looked like a favourable trade.

Now take the same half hour out of a night that was already short, and spend it on the sofa. In short sleepers, shifting 30 minutes from sleep to sitting, to standing or to light activity went together with a slightly higher risk of dying from a condition of the heart and blood vessels.⁠¹ In people who slept enough, standing and light activity went together with a lower risk, but here with a higher one. The trade is the same; only the night the time came from is different.

A third pattern goes the other way. Shifting time from sitting to sleep went together with a more favourable picture, in both regular and irregular sleepers.⁠¹

That standing comes out on the unfavourable side for short sleepers fits with earlier research by the same research group, in which more standing instead of sitting did not go together with better heart and blood vessel health.⁠⁵ Matthew Ahmadi, lead author of that earlier study and co-author of this one, summed it up in a 2024 press release from the University of Sydney:

The key takeaway is that standing for too long does not make up for an otherwise sedentary lifestyle and, for some people, may be a risk to the health of their circulation.

Matthew Ahmadi, deputy director of the Mackenzie Wearables Research Hub at the Charles Perkins Centre and co-author of the new study, The University of Sydney⁠⁷

The authors draw a cautious conclusion: the links between shifts within the 24-hour day and the risk of dying may differ depending on how people sleep.⁠¹ Put simply, the value of half an hour depends on where you take that time from. If you already sleep little, the night may be the wrong place to take that time from.

What the study did not show

These are observational findings.⁠¹ The study saw which shapes of the day went together with which outcomes. It did not show that changing your day changes your risk. The 30-minute swaps are calculated comparisons between people, not changes anyone actually made. Everyone in the study took part in the UK Biobank, so whether the pattern also holds for adults in the Netherlands has not been tested.

We have only read the article’s abstract, and several things are missing from it. The abstract does not say which sleep duration guideline was used or where the cut-off for short sleep lay. Nor does it say how regularity was measured, how long the wearable device was worn or how old the participants were. The abstract gives no result for the third group of causes of death the team examined: death from a group of conditions linked to physical activity. Nor does it give a result for short sleepers who shifted sleep time to vigorous activity. And nothing in the abstract shows that sleeping longer makes someone live longer.

What you can do with this

Three directions follow from the study. None of them is a plan.

First: look at your day as a whole before you add anything to it. A new habit always takes its time from somewhere, and the study suggests that what the habit pushes out matters as much as the habit itself.⁠¹

Second: if your nights are already short, let the time for moving come from sitting rather than from sleep. In short sleepers, giving up sleep went together with the less favourable pattern, while shifting sitting time to sleep seemed favourable for both regular and irregular sleepers.⁠¹

Third: if your rhythm has shifted, think about regularity alongside length. Earlier research in the same cohort found that a regular sleep-wake rhythm predicted the risk of dying better than the number of hours slept.⁠³ Restoring your biorhythm, in other words getting a shifted rhythm back into a steadier shape, is more about nights that resemble each other than about one long night at the weekend.

None of this tells you what your own nights look like: whether you are a short sleeper, how regular your rhythm really is and where you are starting from. That is the open question, and the scan below is one way to make a start on the answer for yourself.