Circadian misalignment

Misalignment is what you get when the time you sleep and the time your circadian rhythm expects you to sleep come apart. The clock does not simply permit sleep at night, it actively promotes wakefulness during the biological day, so sleeping across that period means working against a signal rather than in the absence of one.

The measurable consequence is that hours stop being interchangeable. Eight hours starting at three in the morning is not eight hours starting at eleven at night. Core body temperature is on its way up through the morning, the melatonin window is closing, and daylight keeps reinforcing the wake signal, so the same time in bed produces sleep that is lighter, more broken and shorter.

Who lives with it

Shift workers, most obviously and most severely, since the mismatch is structural rather than occasional. Long-haul travellers, for a few days at a time, which is the version most people have experienced. And anyone with a strongly late or early chronotype whose obligations sit somewhere else, which is the milder chronic form usually called social jetlag.

What actually helps

Light is the only strong lever, because it is the dominant zeitgeber. Where the schedule can be shifted, bright light at the start of your subjective day and darkness at the end of it will move the clock over several days. Where it cannot, the effort moves to protecting the sleep itself: complete blackout, noise control, and consistency across the week rather than flipping between two schedules.

That last point is the one people resist and it is the most useful. A rota worked on the same times every week is easier for a clock to partially adapt to than one that rotates. Tock for night shift workers covers the practical version, and travellers with jet lag the temporary one.

Nothing here is medical advice, and persistent problems from shift work are worth raising with a doctor.