People ask whether time on their phone hurts their training. Direct evidence is thin. The indirect route is better documented: screens at night can push sleep later and make it shorter, and short sleep can lower how well you perform. Here is what each link of that chain rests on.
The short version
- The CDC advises 7 or more hours of sleep for adults aged 18 to 60, and 8 to 10 hours for teens aged 13 to 17.
- A meta-analysis found sleep of 6 hours or less lowered exercise performance by about 7.6% on average, with wide uncertainty.
- Screens near bedtime are linked to shorter, later sleep, but studies cannot show screens are the cause.
How much sleep
The CDC lists 8 to 10 hours for 13- to 17-year-olds, 7 or more for adults aged 18 to 60, 7 to 9 for ages 61 to 64, and 7 to 8 from 65. It adds that quality counts too: trouble falling asleep, waking often, or feeling tired after enough hours are all signs of poor sleep.
What less sleep does to training
A 2022 review pooled 227 outcome measures from 69 publications on how sleep loss (6 hours or less in 24) affects exercise. Performance was lower on average by 7.56% (95% confidence interval 3.13 to 11.9% lower), and every exercise type was affected, including strength, endurance and power. The effect was consistent after deprivation (no sleep, or very little) and after waking early, but not after merely going to bed late. Each hour awake before the exercise task was linked to about 0.4% worse performance. Tasks done in the evening were affected consistently, while morning tasks were largely unaffected. The authors' advice: avoid deprivation and late restriction when you can, and favor morning workouts if you cannot (Craven and colleagues, 2022).
What screens do
In a laboratory study, people who read on a light-emitting device before bed took longer to fall asleep, felt less sleepy in the evening, produced less melatonin, had a later body clock and were less alert the next morning than when they read a printed book (Chang and colleagues, 2015). A systematic review of 67 studies of school-age children and teens found screen time was linked to worse sleep, mainly shorter and later, in 90% of them (Hale and Guan, 2015).
Screens also mean sitting. The 2020 World Health Organization guidelines recommend reducing sedentary time for all ages, while saying the evidence was not strong enough to set a threshold.
A way to test it on yourself
- For two weeks, note your screen time each day and how many hours you slept that night.
- Sort the nights into two groups: after heavier screen days and after lighter ones.
- Compare the average sleep of each group. A gap of half an hour or more across at least a few nights each is worth a closer look. A gap of a few minutes is noise.
Then try a wind-down time, with the phone away from bed, for two weeks and compare again. One person's data cannot show cause, but it can show what is true for you.
How Vital handles this
Vital's Screen Time tab holds minutes you type in from your phone's own Screen Time or Digital Wellbeing screen. It cannot read them automatically. You can log sleep in the daily check-in, and turn on an optional wind-down reminder. Once you have at least 14 days of both, Vital compares sleep after lighter and heavier screen days. It shows a pattern only when the gap is big enough not to look like chance, states how many days it is based on, and never says one thing caused the other.
What we are less sure about
- Cause and direction. The youth review says causation was not confirmed. People who sleep badly may use screens more, and the measures were often self-reported, with error.
- Why screens disturb sleep. The eReader study suggests the short-wavelength light may be the cause, but it said "may". What you do on the screen, and how stimulating it is, may matter too.
- Screens and training directly. We did not find studies that connect evening phone time to the next workout. The chain from screens to sleep to performance is an inference.
- Who was studied. The sleep-loss review was 89% male, mostly short-term sleep loss, and results varied a lot between studies. Long runs of modest sleep restriction are less studied.
- How much is too much. The WHO could not name a safe amount of sedentary time. We cannot name a safe amount of screen time.
Sources
- Centers for Disease Control and Prevention. About Sleep. cdc.gov
- Craven J, McCartney D, Desbrow B, et al. Effects of acute sleep loss on physical performance: a systematic and meta-analytical review. Sports Med 2022;52(11):2669-2690. doi:10.1007/s40279-022-01706-y
- 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. doi:10.1073/pnas.1418490112
- Hale L, Guan S. Screen time and sleep among school-aged children and adolescents: a systematic literature review. Sleep Med Rev 2015;21:50-58. doi:10.1016/j.smrv.2014.07.007
- Bull FC, Al-Ansari SS, Biddle S, et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. Br J Sports Med 2020;54(24):1451-1462. doi:10.1136/bjsports-2020-102955
Written November 5, 2026. This is general information, not medical advice. Talk to a doctor or dietitian before big changes to how you eat or train, especially with a medical condition or a history of disordered eating.