You slept four hours: what to do with today's session

It's half six, the alarm goes off, and your running kit is still folded on the chair, looking at you reproachfully. You went to bed at one because the next episode started all by itself. This morning's question isn't whether you're a disciplined person: it's whether a session done on four hours' sleep will do you any good at all.

One bad night already shows up in your strength

Alberto Pérez-López and Marta Del Val Manzano, researchers at the University of Alcalá, have reviewed in The Conversation what happens to the body when it trains after sleeping badly. Here's the figure that stings: a single four-hour night reduces force production by between 10% and 15% in trained adults. One night. Not a week of excess, not a month of rotating shifts.

Translated into real life: the weight you moved six times on Tuesday jams on the fourth rep today, and you'd swear the bar got heavier. It isn't just you. Alongside the loss of strength, fatigue arrives sooner and the effort feels harder, so your usual pace feels as though somebody steepened the hill a little while you slept. Not much, admittedly.

None of this is a surprise if you're already sold on the idea that sleeping well is the invisible training that pays off most: sleep isn't the stretch when your body switches off, it's the stretch when it rebuilds itself. Skip it and you're training on top of a half-finished building site.

And no, sleeping less doesn't burn more fat

There's an idea doing the rounds that a sleep-deprived body draws more on its fat stores, and that getting up early at the expense of sleep therefore pays off if what you want is to lose weight. The authors stop precisely there, on what happens to fat oxidation when you ride a bike after a bad night, and the conclusion is the one in their headline: it doesn't add up. Even if you gained something on that front, you lose it everywhere else, starting with the quality of the session and continuing with the fatigue you drag into the next day.

Put another way: sleep isn't a currency you can spend on training. When you take an hour off it, that hour doesn't turn up in your athletic current account. It turns up in the overdraft.

What to do with the session, specifically

Adjusting isn't giving up

There's a version of sporting commitment that consists of training always, no matter what, and treating yourself with suspicion the moment an excuse appears. It's exhausting and, on top of that, it works fairly poorly. Real consistency looks more like knowing what today calls for: some days you push, and some days you protect what you've already built.

So if you slept four hours last night, you have permission. Not to settle onto the sofa for three weeks, but to go out for an easy run, enjoy it without checking your average pace, and get to bed early tonight. Thursday's version of you, the one actually doing the intervals, will be grateful.