The Week After The Clocks Change

Twice a year the clock moves an hour and sleep gets worse for several days afterward. The mechanism is a mismatch between social time and the body's own timing.

The internal clock is set by light, not by clocks

Circadian timing is anchored mainly by light reaching the eyes, particularly in the morning. It has no access to what a wall clock says.

Changing the displayed time therefore shifts every appointment, alarm and school start by an hour while leaving the biological schedule where it was.

The body does adjust, but at its own pace, and the adjustment is measured in days rather than in the instant the clocks move.

Spring forward is the harder direction

Losing an hour requires falling asleep earlier than the body is ready to, which is the harder direction because sleep onset cannot be forced by intention.

Gaining an hour asks for staying awake later or waking later, both of which are easier to accomplish and easier to tolerate.

This asymmetry is the same one that makes flying east worse than flying west, and it comes from the same underlying tendency of the human clock to run slightly long.

The morning light shifts too

After the spring change, the hour that used to contain morning light is now dark, so the strongest signal for advancing the clock arrives an hour later in body time.

Evening light meanwhile extends, and evening light pushes timing later, which is the opposite of what the new schedule requires.

Both signals therefore work against the shift for the first week, which is why the adjustment takes longer than the size of the change suggests.

The cost lands on people with no slack

Adults who can shift their own start time absorb the change easily. The people who cannot are the ones for whom it is a genuine disruption.

Teenagers, whose timing already runs late, and shift workers, whose schedules are set externally, both lose an hour they had no margin to give.

Young children are the other group, since their sleep timing is fairly rigid and does not respond to being told what time it now is.

Gradual shifts work better than a single jump

Moving bedtime and wake time by small increments across the several days before the change reduces the size of the mismatch on the day itself.

Seeking outdoor light early on the first mornings after a spring change supports the same shift, because morning light is what advances the clock.

Sleep that stays badly disrupted well beyond a week or two is not the clock change any more, and it is worth raising with a doctor rather than waiting it out.