Screens before bed: what the evidence actually says about blue light, scrolling and sleep

What screen light does to your body clock

Your brain uses light to set its internal clock. Evening light says the day is not over, holds back melatonin (the hormone that signals night) and nudges sleep later. Blue-ish light does this most effectively, hence the worry.

The dose depends on brightness and on how long it lasts. In a laboratory study, 55 young adults spent five hours in evening light. On average melatonin was halved at about 25 lux, roughly a dim living room, and even 10 lux over those five hours delayed melatonin onset by about 20 minutes. Sensitivity varied enormously: 6 lux was enough for the most sensitive person, the least sensitive needed 350. The same phone is a real signal for some and close to nothing for others.

The e-reader study, and what it did not show

The study most often quoted is from Harvard researchers in 2015. Twelve healthy adults spent two weeks in a sleep laboratory, each reading for four hours before bed on five evenings from a tablet at maximum brightness and on five more from a printed book, in random order. The tablet delivered about 32 lux at reading distance, inside the range the lux study found the clock responds to.

Compared with paper, tablet reading suppressed evening melatonin by about 55 percent, delayed the body clock by about an hour and a half, cut REM sleep by roughly 12 minutes and left people sleepier next morning. The effect on falling asleep was real but modest: about 10 minutes longer.

It does not show that an ordinary evening with a phone does the same: four hours at full brightness in a very dim room is far from typical, and 12 people is a small group. Because the text was identical, the difference is down to the lit screen; what it cannot tell you is how that compares with everything else screens do in real life.

The bigger problems: lost time and a switched-on mind

In real life, the light probably matters less than the screen's other effects. A 2025 survey of more than 45,000 Norwegian students found each extra hour of screen use after going to bed was linked with 24 minutes less sleep and 59 percent higher odds of insomnia symptoms. Social media was no worse than anything else: the amount of time, not the activity, tracked with worse sleep. A survey cannot prove direction, and its authors say so: people who cannot sleep also reach for the phone. But they read it as support for displacement, the mechanism a recent review put first.

• Displacement. Screens are built to keep you there, so an hour you meant to spend asleep becomes an hour of scrolling. Staying up past the time you intended, with nothing actually stopping you, is sometimes called bedtime procrastination.
• Arousal. A tense email, an argument in a comment thread, a cliff-hanger, a news alert: anything that gets you thinking keeps your brain in daytime mode, and melatonin cannot do much against a racing mind.

Notifications add a third layer: a buzzing phone wakes you, a silent one keeps part of you on call, and a "quick check" at 3 am becomes half an hour.

Does night mode help?

Less than the marketing suggests. A randomized trial of 167 young adults compared an hour of phone use before bed with Night Shift on, with it off, and no phone at all, over seven nights, sleep measured by wrist monitors. There was no difference between the three. Among those who normally slept more than 6.8 hours, no phone gave the best sleep efficiency, and Night Shift did not close the gap.

A warm tint removes some blue light but leaves the screen bright, and does nothing about the content or the extra hour. Treat a filter as a small bonus, not permission; turning the brightness down likely does more, given how steeply the clock responds to intensity.

A workable evening screen rule for adults

Aim for a rule you can keep, not a ban:

1. Dim. Turn brightness down in the evening and keep room lighting low; warm lamps beat overheads.
2. Distance. A television across the room delivers far less light to your eyes than a phone 30 centimetres from your face.
3. Stop 30 to 60 minutes before bed. The final hour is the window the Night Shift trial tested, and its well-rested participants slept best with no phone in it.
4. Phone out of reach. Charge it across the room or outside the bedroom; a cheap clock can be the alarm.
5. No scrolling in bed. If you wake at 3 am, leave the phone alone; if you cannot settle after 20 minutes or so, get up and do something quiet in dim light.

Give the replacement a fair chance: a paper book, a podcast, a warm shower. Our guides to bedtime routines and sleep hygiene have more ideas.

Teenagers and delayed body clocks

Teenagers tend to be alert later than younger children, and they use devices before bed more than anyone, which is why the Harvard authors flagged them: evening screen light pushes the clock the same way, making a delayed sleep phase likelier. Early school starts cut the morning short, and teenagers need more sleep than adults, not less (see sleep needs by age).

The same rules apply, with three adjustments. Charging phones outside the bedroom helps most, and works best when adults do it too. Bright morning light, ideally outdoors, pulls the clock earlier. And keep it about sleep, not punishment: a teenager who understands why they feel wired at midnight is likelier to cooperate.

When to talk with a doctor

Talk with your doctor if you have had trouble falling or staying asleep most nights for more than a few weeks despite changing your routine, if daytime sleepiness worries you when driving, or if you snore loudly or wake gasping. Persistent trouble has a name, most often insomnia, and it is treatable. A teenager who cannot fall asleep before 2 or 3 am, whatever the household does, may have a clock disorder rather than a phone habit.

In summary

Screen light does suppress melatonin and delay your clock, but for most people an hour or so at ordinary brightness has only a modest effect on falling asleep; long sessions and sensitive sleepers are a different matter. The lost hour and the switched-on mind appear to matter more, and night mode helps less than expected. Dim the screen, stop 30 to 60 minutes before bed, charge the phone out of reach and keep scrolling out of bed.