In 30 seconds
- Ordinary room light in the hours before bed delayed the onset of melatonin in 99% of 116 adults and shortened its duration by about 90 minutes.
- Reading for 4 hours on a tablet before bed delayed sleep by almost 10 minutes and reduced REM sleep compared with a paper book.
- Phone night mode and blue-light filtering glasses have not been shown to improve sleep.
- Experts recommend at least 250 melanopic lux during the day, no more than 10 in the 3 hours before bed and no more than 1 while you sleep.
Light is the signal with the greatest influence on your internal clock. At night, even ordinary home lighting delays melatonin, and screens add to it. During the day the opposite happens: the more natural light you get, the earlier sleep arrives. This is what the studies say, with figures you can put into practice tonight.
Light, your body clock and melatonin
When the light fades, your body starts to produce melatonin, the hormone that marks biological night. Light at night holds it back and shortens that internal night 2.
Not all light has the same effect. Bluer light stimulates the clock most, which is why experts advise light rich in that tone during the day and low in it at night. To measure it they use melanopic lux, a unit that takes that effect on the clock into account 1.
Level of evidence by outcome
| Outcome | Evidence | What the data show |
|---|---|---|
| Home lighting at night delays melatonin | High | It happened in 99% of 116 adults, with room light below 200 lux. |
| Screens before bed | Moderate | Almost 10 minutes longer to fall asleep and less REM when reading on a tablet rather than on paper. |
| More daylight brings the clock forward | Moderate | Melatonin arrived between 1.4 and 2.6 hours earlier after days with plenty of natural light. |
| Light while you sleep | Preliminary | One night with 100 lux raised heart rate and insulin resistance. |
| Phone night mode | No clear effect | No differences in sleep compared with using the phone without it. |
| Blue-light filtering glasses | No clear effect | Mixed results in 6 trials, with very low certainty. |
The rating is our own, based on the number of studies, their quality and the consistency of the results.
The key studies
| Study | Type | Sample | Outcome |
|---|---|---|---|
| Gooley et al., 2011 | Analysis of two laboratory studies | 116 adults aged 18 to 30 | Room light in the 8 hours before bed: melatonin onset delayed in 99% and about 90 minutes less melatonin. 2 |
| Chang et al., 2014 | Crossover trial | 12 adults | 4 hours reading on a tablet versus a paper book: almost 10 minutes longer to fall asleep, less REM and more tiredness in the morning. 3 |
| Wright et al., 2013 | Camping study | 8 volunteers | A week's summer camping with no light but the sun and campfires: more than 4 times more daylight and melatonin about 2 hours earlier. 4 |
| Stothard et al., 2017 | Camping studies | Healthy volunteers | A weekend of camping brought melatonin forward by 1.4 hours; a week in winter, with 13 times more daylight, by 2.6 hours. 9 |
| Brigham Young University, 2021 | Three-group study | 167 young adults aged 18 to 24 | No differences in sleep between using the phone with night mode, without it or not using it before bed. 5 |
| Singh et al., 2023 | Cochrane review | 6 trials, 148 people | Effect of blue-light filtering glasses on sleep undetermined, with very low certainty. 6 |
| Mason et al., 2022 | Laboratory trial | Healthy adults | One night with 100 lux versus 3 lux: higher heart rate during sleep and more insulin resistance the next day. 7 |
116 adults aged 18 to 30
Ordinary home lighting already delays melatonin
Home lighting already counts
You do not need a spotlight. In an analysis of 116 adults aged 18 to 30, spending the 8 hours before bed in ordinary room light, below 200 lux, instead of dim light, delayed melatonin onset in 99% of cases 2.
Its duration was shortened by about 90 minutes: your biological night shrinks. And if that light stayed on during the hours of sleep, melatonin fell by more than half in 85% of the tests 2.
At home, typical evening lighting is around 30 lux. With warm-white LED bulbs, 2,700-3,000 K, that is equivalent to about 10 melanopic lux or less, exactly the maximum recommended for the evening 1.
Melanopic lux
A screen compared with the evening recommendation
Screens
In a study in Boston, 12 adults read from 18:00 to 22:00 for 5 nights on a tablet and another 5 nights from a paper book. With the tablet they took almost 10 minutes longer to fall asleep, had less REM sleep and were more tired in the morning 3.
A screen on its own can give more than 60 melanopic lux in a dark room: six times the maximum recommended for the evening 1. That is why the American Academy of Sleep Medicine advises switching screens off at least 30 minutes before bed 8.
What about night mode?
A Brigham Young University study divided 167 young adults aged 18 to 24 into three groups: phone at night with night mode, phone without it, and no phone before bed. Their sleep was measured with a wrist sensor and, across the group as a whole, there were no differences 5.
Among those who slept about 7 hours, the ones who did not use their phone slept better than the other two groups. The authors' interpretation: what you do on your phone, answering messages or endless scrolling, may matter more than the colour of the screen 5.
What about blue-light glasses?
The 2023 Cochrane review pooled 6 trials with 148 people that measured sleep. Three found improvements and three did not, and the certainty of the evidence is very low. Their effect on sleep is, for now, undetermined 6.
Daylight counts too
University of Colorado researchers took a group of volunteers on a week's camping trip in summer, with no torches at night. They received four times more light than usual during the day and their melatonin started almost two hours earlier, close to sunset 4.
In a second study, a single weekend of camping brought melatonin forward by 1.4 hours. And a week in winter, with 13 times more daylight, brought it forward by 2.6 hours 9.
During the day, experts call for at least 250 melanopic lux at eye level, with natural light as the first option. With ordinary office lighting, the figure often falls short of 200 1.
Sleeping with a light on
In a Northwestern University study, sleeping for a single night with moderate light, 100 lux, instead of near-darkness, 3 lux, raised heart rate during sleep. The next morning, there was more insulin resistance 7.
It is not unusual: up to 40% of people sleep with a lamp, some other light or the television on 7. For sleep, experts call for 1 lux at most, a bedroom as dark as possible 1.
The figures you can use
| When | Recommendation | How to achieve it |
|---|---|---|
| During the day | At least 250 melanopic lux | Natural light whenever you can: outdoors or near a window |
| From 3 hours before bed | No more than 10 melanopic lux | Low lamps with warm light (2,700-3,000 K) and screens away at least 30 minutes beforehand |
| While you sleep | No more than 1 lux | Blinds down or an eye mask; if you need light, dim, amber and close to the floor |
Sources: Brown et al., 2022, for healthy adults aged 18 to 55 on a daytime schedule; American Academy of Sleep Medicine; Northwestern University.
Your light, from day to night
What to do with light, step by step
What to do, step by step
- Natural light in the morning. Go outside or work near a window 1.
- 3 hours before bed, change the light. Switch off the ceiling light and use low lamps with warm light 1.
- Screens away half an hour beforehand, at the very least 8.
- Do not rely on night mode or glasses alone. They have not been shown to improve sleep 5 6.
- A dark bedroom. Blinds, blackout curtains or an eye mask; if you need light, make it dim, amber, away from you and close to the floor 7.
You will find it set out hour by hour in our bedtime routine for better sleep.
Want to know if you are getting the sleep you need? Take our sleep test. And if you sleep badly persistently, see your doctor.
Frequently asked questions
How long before bed should I put my phone away?
At least 30 minutes, according to the American Academy of Sleep Medicine. In one study, reading on a tablet before bed delayed sleep by almost 10 minutes and reduced REM sleep compared with a paper book.
Does phone night mode help you sleep?
There is no evidence that it does. In a study with 167 young adults, using the phone with night mode did not change sleep compared with using it without. What you do on your phone seems to matter more than the colour of the screen.
Do blue-light glasses help you sleep better?
For now it is unclear. A 2023 Cochrane review of 6 trials found mixed results and very low certainty of evidence.
Is it bad to sleep with a light on?
In one study, a single night with moderate light raised heart rate during sleep and worsened the response to insulin the next morning. Ideally, sleep in the dark; if you need light, make it dim, amber and close to the floor.
Sources
- Brown TM, Brainard GC, Cajochen C, Czeisler CA, Hanifin JP, Lockley SW, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biol. 2022;20(3):e3001571. View publication
- Gooley JJ, Chamberlain K, Smith KA, Khalsa SBS, Rajaratnam SMW, Van Reen E, Zeitzer JM, Czeisler CA, Lockley SW. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab. 2011;96(3):E463-E472. View publication
- Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proc Natl Acad Sci U S A. 2015;112(4):1232-1237. View publication
- Wright KP Jr, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED. Entrainment of the human circadian clock to the natural light-dark cycle. Curr Biol. 2013;23(16):1554-1558. View publication
- Duraccio KM, Zaugg KK, Blackburn RC, Jensen CD. Does iPhone night shift mitigate negative effects of smartphone use on sleep outcomes in emerging adults? Sleep Health. 2021;7(4):478-484. View publication
- Singh S, Keller PR, Busija L, McMillan P, Makrai E, Lawrenson JG, Hull CC, Downie LE. Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults. Cochrane Database Syst Rev. 2023;(8):CD013244. View publication
- Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci U S A. 2022;119(12):e2113290119. View publication
- American Academy of Sleep Medicine. Healthy sleep habits. Sleep Education. View publication
- Stothard ER, McHill AW, Depner CM, Birks BR, Moehlman TM, Ritchie HK, et al. Circadian entrainment to the natural light-dark cycle across seasons and the weekend. Curr Biol. 2017;27(4):508-513. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
