What changes deep sleep

Stage N3, or slow-wave sleep. Most of it comes in the first half of the night.

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.

AlcoholAlcohol can help you fall asleep, then breaks up the second half of the night, cuts REM sleep, raises your heart rate and lowers HRV. Less, and earlier, is better.
- Evidence: Strong
- Medium effort
- Free
- Works: tonight
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
How sleep worksTwo systems decide when you get sleepy and how deeply you sleep. Almost every tip on this site works through one of them.
- Evidence: Strong
- Low effort
- Free
- Works: 1 week
Sleep stages and cyclesLight, deep and REM sleep, why deep sleep comes early and REM comes late, and why "90-minute cycles" aren't precise.
- Evidence: Strong
- Low effort
- Free
- Works: tonight
Building sleep pressureBe properly tired at bedtime. Stay awake long enough, move during the day, don't go to bed too early, and keep naps and caffeine from draining the pressure.
- Evidence: Some
- Low effort
- Free
- Works: tonight
Darkness and eye masksMake the bedroom as dark as you can, with blackout curtains or an eye mask. Experts recommend under 1 lux at the eye while you sleep.
- Evidence: Some
- Low effort
- Low cost
- Works: tonight
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
Morning lightGet outside in daylight soon after you wake up. It sets your body clock earlier, so you get sleepy at a sensible time.
- Evidence: Some
- Low effort
- Free
- Works: 1–2 weeks
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
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
Bedding materialsCotton, linen, wool, silk, down and synthetics compared. Few good studies exist; choose by whether you sleep hot or cold, and don't expect mite-proof covers to fix allergies.
- Evidence: Weak
- Low effort
- Medium cost
- Works: tonight
Duvet weight and layeringMatch your duvet's warmth (tog) to the season, and use layers so you can adjust through the night. Weighted blankets have some evidence, but mainly in people with psychiatric conditions.
- Evidence: Weak
- Low effort
- Medium cost
- Works: tonight
Evening mealsEat dinner 3–4 hours before bed if you can (at least 2–3), keep it moderate in size, and go easy on spice and fat if you get reflux. Most studies on food and sleep are small.
- Evidence: Weak
- Low effort
- Free
- Works: tonight
GlycineThree grams of glycine before bed improved how people felt about their sleep in a few tiny Japanese trials, most run by a glycine maker. Promising, but far from proven.
- Evidence: Weak
- Low effort
- Low cost
- Works: 1–3 nights in trials
HumidityAim for 40–60% relative humidity in the bedroom. Hot and humid is the combination that clearly hurts sleep; very dry air mostly affects your eyes, nose and throat.
- Evidence: Weak
- Low effort
- Low cost
- Works: tonight
SaunaMany regular sauna users say it helps them sleep, but no good trial has tested it. If you enjoy it, finish an hour or two before bed, drink water and skip the alcohol.
- Evidence: Weak
- Medium effort
- Medium cost
- Works: tonight