The clocks change: what it does to your body clock

Why one hour costs more than an hour

Your body clock is set by light hitting the eye, principally in the morning, and it drifts back into alignment at a limited rate — commonly cited as about an hour a day, less for people with a late chronotype. A clock change moves your social schedule instantly and your biology slowly, which is why the mismatch takes several days to close rather than one night. It is the same mechanism as jet lag, in a milder dose and without the flight.

The two directions are not equal. The spring shift takes an hour away and is the one associated in population data with a short-term rise in heart attacks, strokes and traffic collisions in the days that follow — small relative increases on top of ordinary rates, but consistent enough across countries to be taken seriously. The autumn shift gives an hour back and is much gentler, though it comes with its own annoyance: waking an hour early for several days, and an evening that gets dark uncomfortably fast.

The autumn hour you do not get to keep

The extra hour is credited to the night of the change and spent almost immediately. Most people wake at their old body-clock time, which is now an hour early on the new clock, and by the second or third night the sleep-onset time has usually not moved to match. The net effect for a lot of people is one longer night followed by three or four slightly shorter ones — which is why the week after the autumn change often reads worse in the data than the week before it, despite the extra hour.

Shifting on purpose beats waiting it out

You can absorb most of it in advance. For three or four days before the change, move bedtime and wake time by fifteen minutes a day in the direction of the shift — later for autumn, earlier for spring. Then use light deliberately: bright light in the morning pulls your clock earlier, evening light pushes it later, and this is by far the strongest lever you have. Keep meal times and any training moving with the schedule too, since both act as weaker secondary time cues. Fixing your wake time and getting outside soon after it does more than anything you can do at bedtime.

What to expect in the data

A visible jump in your sleep midpoint, a few days of reduced sleep regularity, and — for some people — slightly more fragmented nights while the shift settles. Resting heart rate and HRV may wobble for two or three nights, in the same way they do after any disturbance to timing. None of it means much on its own, and a readiness score that dips the Monday after a clock change is describing a scheduling event rather than your health.

The right timescale here is the week, not the day. If your numbers have not returned to your own normal by five or six days after the change, the clocks are unlikely to be the cause any more — look at what else changed that week, and remember that the shift usually coincides with the point in autumn where morning light disappears from the commute.

Reading it against your own baseline

Mark the change date, then compare the week before and the week after on the things timing actually affects: sleep midpoint, regularity, total sleep and wake time. Vitra keeps that comparison local, computed on your own machine against your own history, so a bad Tuesday sits inside a visible pattern rather than looking like a verdict. Most people are back to their own normal within a week; if a fortnight passes and your timing has not settled, that is worth taking seriously rather than blaming on the hour.

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