What Reading on a Tablet Before Bed Does to Your Sleep

Twelve healthy adults spent two weeks living in a tightly controlled research unit, reading either a printed book or an iPad for four hours each evening before bed, then switching to the other for a second week. Reading on the tablet delayed the evening rise in melatonin, the hormone that signals the body it's time to wind down, by more than an hour compared with reading a printed book under dim light. Participants on the tablet also took longer to fall asleep, spent less time in REM sleep, and were measurably less alert and more sluggish the following morning, despite getting the same eight hours in bed.

Four hours of reading before bed is a long stretch, longer than most people would sit with a book, but the researchers pointed out that once other evening screen use is factored in, phones, laptops, browsing, that total exposure isn't unusual for many people. The study measured a single controlled scenario rather than everyday habits, and only 12 people took part, but the mechanism it demonstrates, that light-emitting screens close to bedtime measurably push the body's clock later, has since been replicated widely and underpins most current guidance on evening screen use.