Screens and Sleep: What Actually Matters

Blue light became the villain because it's easy to sell filters for. The evidence is less tidy: light exposure matters, but the bigger levers are when you use the screen and how activating what's on it is.

Three mechanisms, ranked

• Displacement: the hour you spent scrolling was an hour you weren't asleep. This is the largest and least interesting effect.
• Arousal: an argument in a comment section, a work email, or a cliffhanger episode raises alertness at exactly the wrong time.
• Light: real, dose-dependent, and much smaller than the first two for typical phone brightness at arm's length.

What to change first

Pick a device curfew you can hold — 30 minutes before bed is a better rule than 90 minutes you abandon on Thursday. Then change the content, not just the colour temperature: reading is not the same input as your inbox.

Charge the phone outside the bedroom if you can. If it's your alarm, buy a clock. It is the cheapest sleep intervention available.

You don't need a perfect evening routine. You need the phone to stop being the last and first thing you touch.