Circadian rhythm

A master clock in the hypothalamus keeps time from light, meal times and a consistent wake time, and it moves more slowly than a flight schedule does. These guides cover what shifts it, in what order, and how fast.

The jet lag guide is built on the chronobiology: roughly 10 to 20 minutes of outdoor light within half an hour of waking at the destination, local meal times from arrival day, and the asymmetry that makes eastward travel the harder direction to adjust to. The 30-day reset applies the same cues at home and insists on their order, because morning light and a fixed wake time do most of the work and everything else only starts helping once both are in place.

The weekend guide is the awkward one, and it stays awkward. Large UK accelerometer analyses disagree with each other about whether catching up protects you, a 36-person lab study found metabolic changes persisting after two recovery nights, and what the evidence supports more consistently is that recurring swings in timing carry their own risk. Regularity and duration are two measurements, not one.