By Tim Olds

There’s nothing more democratic than time. Donald Trump and Hillary Clinton, Bill Gates and the Dalai Lama, Usain Bolt and Serena Williams all have exactly 1,440 minutes in their day, and the rip of time sweeps us all towards the future at exactly 60 minutes per hour.

Imagine this: you decide you need to get fit, so you determine to go swimming for half an hour each day. You need to find time to get into your swimmers, get to the pool, do your workout, get home, have a shower and get changed. Let’s say an hour all up. Where does the time come from? It’s not magicked out of nothing. Time, like water, is incompressible and inextensible. You could get up earlier or go to bed later, skive off work a little earlier, have a shorter shower, skip some social media time, forego intimacy with your beloved, get some takeaway instead of cooking. What would you do? What do people actually do?

I know the answer because a few years ago we did a little research project. We got 100 people and asked two-thirds of them to start an exercise program, in their own time, requiring either two-and-a-half or five hours a week. The other third were told to just live their lives as usual.

We compared changes in time use between the two groups. So, where did the time come from?

It turns out that the extra hours came from the three deep time reservoirs of modern life: TV, sleep, and household chores. When they needed to squeeze out some extra minutes and hours, our participants slept less, watched less TV, and let the house go to rack and ruin. Unfortunately, at the end of the seven-week program, their time use flowed back to exactly what it was before: when we have time on our hands, we catch up on sleep and TV, and put the house back in order. The tidal drag of routine eventually pulls us back into our quotidian rhythms.

And so we are constantly ballasting bits of time against others, trying to balance our lives. We’re told to get 8 hours’ sleep, at least 30 minutes of physical activity and not to spend too much time sitting down. The problem is this: if you do more of one thing, you have to do less of another. If I decide to sleep an extra hour, something has to go. What should I sacrifice? Physical activity? TV? Socialising? And what’s the temporal exchange rate? To get the same reduction in depression, for example, as I would get from 30 minutes walking each day, how much extra sleep would I need? How much less time would I have to spend watching TV?

Retirement is a nice life stage to study this, because one day you wake up and no longer have to go to work. You feel like you’re jetsam on the banks of the river of time. How do you fill up the empty hours? It turns out that you fill them up with about 40 minutes more sleep, 40 minutes more TV and an hour’s more chores – those time reservoirs again – plus a little extra reading and socialising.

We measured 100 older adults from six months before retirement to a year after, and had a look at how changes in their time use predicted changes in their mental health and well-being. Filling up the empty hours with either more physical activity or more sleep was universally beneficial. But the exchange rate between sleep and physical activity was different depending on which aspect of mental health you were looking at. For overall well-being an extra 45 minutes of sleep was as good as 60 minutes of walking. I’ll take the sleep, thanks. But for stress, you need to sleep an extra two-and-a-half hours to get the same benefit as just 20 minutes of walking. Easier to walk.

The big question is: What’s the best balance, just the right mix for health and well-being, not too much and not too little of anything? What’s the right trade-off? What’s the Goldilocks Day?

We have some clues about this, too. We measured how 1,000 11-year-old kids and their parents used their time over a number of days, using activity trackers on their wrists and 24-hour recalls. We measured every aspect of their health and well-being. We took so much blood, tissue, hair, urine, faeces, and fingernail clippings that they left the lab a couple of kilos lighter. We measured blood pressure, blood glucose, blood lipids, kidney function, liver function, immune function, body fat, hearing, quality of life, biological age, literacy, numeracy... and then we asked what mix of activities predicted the best possible health.

It turns out that the Goldilocks Day is pretty much the same for almost all those different health outcomes, from quality of life to blood pressure. For adults, the best mix is 6-9 hours of sleep, no more than 7 hours of sitting, and 9-12 hours either standing or moving. Kids have it much easier: the Goldilocks Day is 8.5-11.5 hours of sleep, no more than 8 hours sitting, and 5-7 hours standing or moving. With kids, however, there is a catch. For the best results in school, the best day is 11-14 hours sitting and only 1-2 hours of standing or moving. So there are time trade-offs here, too. In 15-year-old boys, the best amount of sleep for academic performance is seven and three-quarter hours. For mental health it is eight and three-quarter hours. What do you want your son to be: an A-grade student who is depressed, or a B-grade student who is happy?

You would think that the fluidity of the modern world would make these trade-offs a bit easier. We are no longer tied to the iron regimen of TV schedules or the workplace. We can defer our TV viewing for whenever we like. We can work at home, in the evenings or on the weekend. We can do our shopping online. We seem to live in a world of liquid time, where we can trade off the present against the future. But we’re all carried along by the ebbs and flows of the river of time, and no amount of revolutionaries shooting the clocks, creating new calendars or declaring Year Zero will change that. Time is a Möbius strip, where sacrifices blend imperceptibly into compensations. We’re always running out of time, and time is slipping away. All we can do is to sit and stare out over the infinite horizons of the great ocean of time, and do our best to manage it.

Tim Olds is Professor of Health Sciences at the University of South Australia. He holds two PhDs, one in French studies from the University of Sydney, another in exercise science. His research interests have been in mathematical modelling of cycling performance, anthropometry, and trends in fitness, fatness, physical activity and food intake. He was Project Director for the Australian National Nutrition and Physical Activity Survey.