How does light affect sleep?
Besides the rods and cones you see with, your retina has light-sensing cells whose main job is telling your body clock what time it is. They respond most strongly to blue light. When researchers mapped which colors suppress melatonin, the hormone that makes you sleepy, the strongest were blue wavelengths around 446–477 nm.1 White LEDs, including most screen backlights, peak right in that range.
Is screen time before bed really a problem?
In a controlled sleep-lab study, people read either on a light-emitting e-reader or a printed book for several hours before bed, on several evenings in a row. After the e-reader, they released less melatonin, their body clocks shifted later, they took longer to fall asleep and they were less alert the next morning.2
Does room light count too?
Yes. Ordinary room light in the hours before bed delayed the rise of melatonin and shortened the time it was released by about 90 minutes, compared with dim light.3 What adds up is the total amount of bright, blue-rich light you see in the evening.
Do night mode and blue-light glasses help?
Night mode tints the picture warmer in software, but the LED behind it still has its blue peak, and any PWM flicker stays the same. For blue-light glasses, a 2023 Cochrane review found the evidence on sleep too uncertain to draw conclusions.4 Changing the light source itself is the more direct fix.
Who is most affected?
People whose body clocks already run late feel it most. In one study, 78% of adults with ADHD had chronic trouble falling asleep, with melatonin arriving later in the evening than in people without ADHD.5 Read more about ADHD and screens and night owls and delayed sleep phase.
Children react even more strongly to evening light than adults; see kids, screens and sleep.
Better evening screen habits
- Dim screens and room lights in the last hours before bed.
- Use warm, low-blue light in the evening.
- Put screens away for the last half hour to hour before sleep if you can.
- Get bright daylight in the morning to anchor your body clock.