Training at night? The four-hour rule

You leave work at seven, get home, get changed, and by the time you've finished your intervals it's nearly ten. You eat, you shower, you're in bed by half eleven… and there you stay, wide awake, legs still humming. If that sounds familiar, there's one number that will reorganise your week: four.

Four million nights for a one-line rule

A study published in Nature Communications tracked 14,689 physically active people for a year using a biometric wristband, gathering more than four million recorded nights. This isn't a survey or a lab with twenty volunteers: it's ordinary people training and sleeping at home, measured night after night for months.

The pattern that emerges is uncomfortably clear. Training late and hard delays the moment you fall asleep, shortens your sleep and worsens its quality. But — and here's the good news — sessions that finish four or more hours before bedtime aren't associated with any change at all. None. The question isn't whether you train at night: it's how hard you go and what time you stop.

Your cut-off is bedtime minus four

The rule fits into a subtraction. If you get into bed at 11.30pm, the demanding session should be finished by 7.30pm. If you turn in at 10.30pm, make it 6.30pm. And yes, I know what you're thinking: on a weekday that's simply impossible. That's what the second half of this article is for, but it helps to see the timetable first.

An honest note about that table: what the study measures is the four-hour threshold and the effect of easing off. The middle bands are the sensible application of that rule to a real evening, not a separate measurement. Use them as a guide, not as gospel.

If you can't make the cut-off, ease off (but don't cancel)

This is my favourite part of the study, because it disarms the perfect excuse. Swapping a very demanding session for an easy one two hours before bed brought sleep onset forward by around 36 minutes and lengthened sleep by 5.4%. In other words: when you can't make the four-hour margin, the answer isn't to stay on the sofa, it's to train more gently.

Thirty-six minutes doesn't sound like much until you multiply it by the four evenings a week you train late. Now we're talking about more than two hours of sleep recovered every week without giving up moving, which is exactly the invisible training that pays off most and the one most people skip.

The plan you'll actually keep

If evenings are the only time you can train, your week doesn't have to be worse: it has to be arranged differently. Concentrate the hard work on the days you get home early or at the weekend, and save the tight evenings for the sessions that don't leave your body switched on. This isn't a plan of bare minimums, it's a plan that survives October, which is when plans tend to collapse.

And if one night you overrun on both the clock and your heart rate, nothing terrible happens: the study describes patterns across a year, not punishments for a single session. Anyone can absorb the odd late one. What doesn't get absorbed is doing it every Tuesday for six months.