What changes total sleep

How long you sleep.

Every page below says it can change this metric. Strongest evidence first. Each page's "Effect on your sleep score" section says how much and how sure we are.

Caffeine cutoffHave your last coffee at least 9 hours before bed. Caffeine cuts sleep and deep sleep even when you fall asleep fine.
- Evidence: Strong
- Low effort
- Free
- Works: tonight
What "good sleep" meansEnough hours, falling asleep without a fight, few wake-ups, regular times, and feeling alert in the day.
- Evidence: Strong
- Low effort
- Free
- Works: 1 week
ChronotypeYour chronotype is how early or late your body clock runs. It's partly genetic and changes with age, but light can move it by an hour or two.
- Evidence: Some
- Low effort
- Free
- Works: 1–3 weeks
Exercise timingRegular exercise helps sleep a little. Evening exercise is fine for most people; just finish hard sessions more than an hour before bed.
- Evidence: Some
- Medium effort
- Free
- Works: a few weeks
How accurate are wearables?Sleep trackers are good at telling sleep from wake and at total sleep time. They are weaker at spotting time awake and at sleep stages.
- Evidence: Some
- Low effort
- Free
- Works: today
Jet lagJet lag is your body clock running on home time after you fly. Timed light, a shifted sleep schedule and, where allowed, melatonin can cut it short.
- Evidence: Some
- Medium effort
- Low cost
- Works: 2–7 days
MelatoninMelatonin is better at shifting your body clock than at knocking you out. For sleep onset it saves about 7 minutes on average. Low doses work; labels are often wrong.
- Evidence: Some
- Low effort
- Low cost
- Works: 1–3 days for timing shifts
NapsA short nap of 10 to 20 minutes in the early afternoon can boost alertness without hurting your night. Long or late naps cost you deep sleep and make it harder to fall asleep.
- Evidence: Some
- Low effort
- Free
- Works: today
Noise and earplugsNight-time noise disturbs sleep even when you don't fully wake. Block it at the source or wear earplugs; the evidence for white noise is very weak.
- Evidence: Some
- Low effort
- Low cost
- Works: tonight
Room temperatureKeep the bedroom between about 18 and 25 °C (64–77 °F), and use bedding to fine-tune. Heat harms sleep more clearly than mild cool.
- Evidence: Some
- Low effort
- Low cost
- Works: tonight
Screens and blue lightThe light from your phone matters less than you've heard. What you do on it, and how late it keeps you up, matters more. Dim the room, and put the phone down when you're sleepy.
- Evidence: Some
- Medium effort
- Free
- Works: tonight
Social jet lagSocial jet lag is the gap between when you sleep on workdays and on days off. Shrink it by keeping your wake time steady and getting morning light.
- Evidence: Some
- Medium effort
- Free
- Works: 1–2 weeks
Waking at 3 a.m.Brief wake-ups are normal. What to do when you can't get back to sleep, and when it's a sign of insomnia.
- Evidence: Some
- Low effort
- Free
- Works: 1–2 weeks
What sleep scores measureWhat goes into the sleep and readiness scores from Oura, WHOOP, Apple Watch and Garmin, according to each company's own documentation.
- Evidence: Some
- Low effort
- Free
- Works: today
Which habits change which metricA table of common sleep habits against the numbers your wearable shows, with the direction of each effect, how sure the research is, and where nobody has looked.
- Evidence: Some
- Low effort
- Free
- Works: tonight to 2 weeks
MagnesiumMagnesium might shorten the time it takes older adults to fall asleep a little, but the trials are few, small and low quality. Stay at or under 350 mg a day from supplements.
- Evidence: Weak
- Low effort
- Low cost
- Works: 2–8 weeks in trials
Mattress firmnessMedium-firm is the best-supported choice, especially if you have low back pain. The evidence is real but low quality, so your own comfort over a few weeks matters most.
- Evidence: Weak
- Medium effort
- High cost
- Works: about 4 weeks
Tart cherryTart cherry juice added sleep time in a few tiny trials, but the evidence is weak, several trials were industry-funded, and the juice is high in sugar.
- Evidence: Weak
- Low effort
- Low cost
- Works: 1–2 weeks in trials