Screen Time Cutoff Calculator
Bright screens in the last hour before bed hold your body clock open, delay melatonin, and cost you sleep. But not every screen is equal: a phone held close at full brightness is far more disruptive than a dim TV across the room. Enter your bedtime, device, and brightness to get a personalised screen-off window, presented as a range because people differ in how strongly evening light affects them.
How screens affect sleep
Your body times sleep partly by light. As evening falls and light dims, the pineal gland releases melatonin, which lowers alertness and prepares you for sleep. Bright, blue-enriched light, exactly what phones, tablets, and laptops emit, tells your brain it is still daytime, suppressing melatonin and nudging your internal clock later.
In a controlled study, Chang et al. (2015) had participants read on a bright light-emitting device for four hours before bed. Compared with reading a printed book, melatonin was suppressed by about 55%, circadian timing was delayed by roughly 1.5 hours, and participants took longer to feel alert the next morning. The effect is dose-dependent: brighter, closer, and bluer screens do more (Gringras et al., 2015).
How the cutoff is calculated
The calculator starts from a base buffer that reflects how much light a device typically delivers to your eyes: a phone or tablet held close gets the largest buffer, a laptop or monitor at arm's length a bit less, and a TV across the room the smallest. It then scales that buffer down if your screen is dim and again if you use a warm filter, since both lower the light dose. The result is shown as a fifteen-minute window rather than a single time.
This is a practical model built on the direction and rough size of the research, not a precise personal prediction. Because sensitivity to evening light varies several-fold between individuals (Phillips et al., 2019), treat the window as a cue to start winding down.
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Do night filters help
Warm-light modes such as Night Shift and Night Light shift the screen toward amber and usually lower brightness, which reduces blue content and eases melatonin suppression. That is why the calculator shortens the recommended buffer when a filter is on. But filtering does not make a screen harmless: the total amount of light still counts, and Gringras et al. (2015) found that filtering alone did not remove the sleep effect. Dimming the display and stopping earlier both do more than a filter on its own.
Building a wind-down
Use the cutoff as the start of a wind-down rather than a sudden lights-out. When the window arrives, switch to low-light, low-arousal activities: a printed book, an e-ink reader without a backlight, a warm shower, stretching, or music. Lowering the lights in the room reinforces the same signal your screens were fighting. If you must stay on a device, dim it, enable the warm filter, and hold it farther from your face. If screen habits are tangled up with racing thoughts or persistent trouble sleeping, that is worth raising with a doctor.
Related calculators
- Caffeine Cutoff Calculator the other big evening cutoff, based on caffeine half-life
- Melatonin Timing Calculator when to take melatonin, and why light exposure works with it
- Sleep Hygiene Quiz score your overall bedtime habits and find the weak spots
Frequently asked questions
How long before bed should I stop looking at screens?
A common guideline is to stop using bright screens roughly one to two hours before bed. The right buffer depends on the device and how bright it is: a phone held close to your face at full brightness does more than a TV across the room. This calculator scales the window by device, brightness, and whether you use a warm filter, and presents a range rather than a single time because light sensitivity varies a lot between people.
Why does screen light before bed affect sleep?
Light in the evening, especially the blue-enriched light from screens, suppresses melatonin, the hormone that signals your body it is time for sleep, and can push your body clock later. Chang et al. (2015) found that reading on a bright light-emitting device before bed suppressed melatonin by about 55% and delayed circadian timing by roughly 1.5 hours compared with a printed book, and left readers less alert the next morning.
Do night mode and blue-light filters actually help?
They help but do not solve the problem. Warm filters like Night Shift or Night Light reduce the blue content and overall intensity of the screen, which lessens melatonin suppression. But brightness and the sheer amount of light still matter, and Gringras et al. (2015) note that filtering alone does not remove the effect. Dimming the screen and, most of all, stopping earlier do more than a filter by itself.
Is it the content or the light that keeps me awake?
Both matter, through different routes. The light delays your body clock and suppresses melatonin. The content, an engaging show, a stressful email, or an endless feed, raises mental and emotional arousal that makes it harder to wind down. This calculator addresses the light side. Choosing calmer activities in the last hour helps the arousal side.
What should I do instead of screens before bed?
Anything low-light and low-arousal works: reading a printed book or an e-ink reader with no backlight, a warm shower, stretching, listening to music or a podcast, or dimming the lights and letting your eyes adjust. The goal is to give melatonin an uninterrupted head start and let your body wind down. If you must use a device, dim it, enable a warm filter, and hold it farther away.
Created and maintained by Reede Taylor · fact-checked against the sources below · Last reviewed July 15, 2026
Educational information, not medical advice. Consult a healthcare provider for sleep disorders or before taking any supplement.