Five chapters, one winter of nights.

1,412 people, 38,114 nights, and the first surprise: bedtime is a range, not a time.

Two in three took a phone to bed. On those nights they slept 41 minutes less, and the minutes went missing at the start.

Six groups, six kinds of night. New parents wake most, night-shift workers sleep least, retirees sleep longest but lightest.

On Friday and Saturday nights the median bedtime moved 1h 12m later. Most people spend Monday recovering from a flight they never took.

We tested what volunteers actually changed. The winners were boring: a fixed wake-up, a cold dark room and a phone in the hall.