Evening screen light can delay sleep: in a sleep lab, people reading on a light-emitting tablet before bed took longer to fall asleep and made less melatonin than when reading a printed book. Putting screens away at least 30 minutes before bed and dimming them in the evening matters more than blue-light glasses.
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How To Use The Blue Light Exposure Calculator
- Enter your bedtime and when you put screens away. If you scroll in bed, use the time you last look at the screen.
- Describe your evening screen use. How long, which screen and how bright, in the 3 hours before bed.
- Describe your room lighting. Overhead lights add to the light reaching your eyes, whatever the screen.
| Result | What it means |
|---|---|
| Evening light score | Low, moderate or high, on a 0 to 9 scale |
| Screen-free time before bed | The gap between your last screen and bedtime |
| Screens away by | 30 minutes before your bedtime |
| Biggest factor and best first change | The single change likely to help most |
What Is Blue Light And How Does It Affect Sleep?
Blue light is the short-wavelength part of visible light. Daylight is full of it, and so are the LED screens of phones, tablets and laptops. The body clock uses light reaching the eyes to set its time, and light in the evening tells the brain to make less melatonin, the hormone that helps you feel sleepy.
A well-known sleep lab study compared reading on a light-emitting tablet with reading a printed book before bed. With the tablet, people took longer to fall asleep, felt less sleepy in the evening, made less melatonin, had a later body clock and were less alert the next morning.
Screens are only part of the picture: bright room lights add to the effect, and what you do on the screen can keep you alert as well. If your sleep timing keeps drifting later, see why your sleep schedule keeps failing.
How Does The Blue Light Calculator Work?
It adds up points for the habits most linked to evening light reaching your eyes, then names the one that adds the most.
Score = Screen-free time (0-3) + Screen time (0-3) + Close screen (0-1) + Brightness (0-1) + Bright room (0-1)| Factor | Points |
|---|---|
| Screen-free time before bed | 60 min or more 0; 30 to 59 min 1; under 30 min 2; screens in bed 3 |
| Screen time in the last 3 hours | None 0; under 1 h 1; 1 to 2 h 2; 2 to 3 h 3 |
| Screen held close (phone, tablet, laptop) | 1 |
| Normal or full brightness | 1 |
| Bright overhead room lights | 1 |
0 to 2 is low, 3 to 5 moderate and 6 to 9 high. The score is our plain summary to help you compare habits, not a validated clinical measure.
Worked Example: Scrolling A Phone Until Bedtime
| Factor | Points |
|---|---|
| Phone put away at 11:00 PM, bed at 11:00 PM | 2 |
| 2 to 3 hours of screen time | 3 |
| Phone held close | 1 |
| Full brightness | 1 |
| Bright overhead lights | 1 |
| Total | 8 of 9: high |
Putting the phone away at 10:30 PM and switching to a lamp would drop the score to 6. Cutting evening screen time to under an hour as well would bring it to 4, and turning brightness down to 3.
Blue-Light Glasses vs Dimming And Timing: What Works?
| Change | What the evidence says |
|---|---|
| Blue-light filtering glasses | A Cochrane review of 17 trials found they probably make no difference to sleep quality |
| Putting screens away before bed | Sleep hygiene advice is at least 30 minutes before bedtime |
| Reading a printed book instead | Led to faster sleep and more melatonin than a light-emitting tablet in a lab study |
| Dimming room lights and screens | Less light reaches the eyes, the main signal to the body clock |
Night mode and warm screen settings change the colour of the light but not always how bright it is. Turning brightness down and putting the screen away earlier are the more reliable steps.
Factors That Change How Evening Light Affects You
Timing
Light close to bedtime matters most, because it falls in the evening part of your body clock when light pushes the clock later.
Brightness And Distance
A bright phone a few inches from your face sends more light to the eyes than a dim TV across the room.
What You Do On The Screen
Messages, games and news can keep the brain alert, separate from the light itself.
Daytime Light
Plenty of daylight earlier in the day helps anchor the body clock; check your timing with the circadian rhythm calculator.
Who Should Use A Blue Light Calculator?
- People who take a long time to fall asleep after evening screen use.
- Night owls whose bedtime keeps drifting later.
- Parents of teenagers, who often use phones late into the night.
- Anyone deciding whether to buy blue-light glasses, who can see what is likely to help more.
Common Mistakes With Screens And Sleep
Relying On Glasses Or Night Mode Alone
Filters change colour, not timing or brightness. The evidence for glasses improving sleep is weak.
Scrolling In Bed
It keeps light close to your eyes right up to sleep and links the bed with being awake.
Ignoring Room Lights
Bright overhead lights add a lot of evening light, even with screens off.
Swapping The Phone For A Laptop
Any bright screen held close has a similar effect. A paper book or audio is a better wind-down. The 10-3-2-1-0 sleep rule builds this into a routine.
Accuracy And Limitations
The score ranks your habits but does not measure light. It cannot tell how bright your screen or room really is, or how sensitive you are to light.
What It Does Well
- Shows which habit matters most
- Gives a clear screens-off time
- Explains what glasses can and cannot do
What It Cannot Do
- Measure light in lux
- Account for individual light sensitivity
- Diagnose a sleep problem
If you still struggle to sleep after cutting evening screens, read sleep hygiene 101 or talk to a doctor.
How We Calculate Your Evening Light Score
Blue Light Exposure Calculator FAQs
Does blue light from phones affect sleep?
Yes, evening light from screens can delay sleep. In a lab study, reading on a light-emitting tablet before bed delayed sleep and reduced melatonin compared with a printed book.
How long before bed should I stop using screens?
At least 30 minutes before bed is common sleep advice. An hour or more gives your body clock a clearer signal.
Do blue-light glasses help you sleep?
Probably not. A Cochrane review of 17 trials found blue-light filtering glasses probably make no difference to sleep quality.
Does night mode stop blue light?
Night mode warms the colour but the screen can still be bright. Turning brightness down and putting the phone away earlier help more.
Is TV better than a phone before bed?
A dim TV across the room sends less light to your eyes than a bright phone held close, but both can keep you up late.
Does blue light damage your eyes?
The Cochrane review found no clear evidence that blue-light filtering lenses protect eye health. Screen time can cause tired eyes from focusing.
What should I do instead of screens before bed?
Read a printed book, listen to audio or music, stretch, or prepare for the next day under dim lamps.
Why do I feel more awake after using my phone in bed?
Bright light close to your eyes and engaging content both keep the brain alert and push back the sleepy signal.
Sources
- Chang et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness (PNAS, 2015) (screen light before bed delays sleep and melatonin)
- Cochrane: Blue-light filtering spectacles probably make no difference to eye strain, eye health or sleep quality (17 trials)
- A good night rest: a contemporary review of sleep and cardiovascular health (2025) (limit devices at least 30 min before bed)
- NIGMS: Circadian rhythms (light controls melatonin via the master clock)
Related Calculators
For bedtimes and wake-up times in both directions, use the main sleep calculator. Explore every sleep timing, sleep need and sleep habit tool in one place.
All sleep toolsThis calculator compares evening light habits for general education. It is not medical advice or a light measurement. Talk to a doctor about ongoing trouble sleeping. Spotted an error? Let us know.