Why does my blood oxygen drop at night?
On this page
Because it’s supposed to. You breathe a little less in sleep, so your blood oxygen dips, lowest around 1 am. In 33,080 Apple Watch wearers the night averaged 95.4 against 96.2 by day. One low dip is normal. Dips that keep coming, with loud snoring or breathing pauses someone has seen, are for your doctor.
What we’d do
Compare last night with your own usual nights, on the same device at the same elevation. If one night looks low, run three more the same way. If the lowest point keeps dropping under 90, or someone has seen you stop breathing, ask your doctor about a sleep study.
Why does sleep lower it?
You move less air. In a 1985 study, researchers measured six healthy men through the night. They breathed about 6.3 liters of air a minute awake, 5.7 in deep sleep and 5.4 in dream (REM) sleep, and their blood oxygen slipped from 97.3 to 96.5 and then 96.2. The researchers found the drop in breathing explained the whole fall.
Think of a car idling at a red light. The engine turns slower, it uses less, and nothing’s wrong with it. A car that stalls at every light is a different story, and that’s what doctors look for at night: breathing that stops and starts over and over.
When in the night is it lowest?
Around 1 am. The Apple Heart and Movement Study gathered about 72 million blood oxygen values from Apple Watch wearers in 2021, and the lowest values of the whole day came around 1 am and the highest around 11 am. Apple says slightly lower values while sleeping are expected, and some people see values below 95.
How low does a healthy night go?
Lower than most people expect. In 350 people with normal overnight sleep studies, the lowest point of the night averaged 90.4, and most landed between about 84 and 97. Sex, race and weight made no difference, and people over 60 ran a little lower. An 88 on your watch at 3 am, once, is something healthy sleepers see too (is 88 normal while sleeping?).
What makes the dips deeper?
| What | How much | Who was studied |
|---|---|---|
| Sleeping higher up | About 1.2 points off the night average for every 3,300 feet | 33,080 Apple Watch wearers |
| Drinks in the evening | About 0.6 points off the average and 1.25 off the lowest point | 14 trials, 422 people |
| The night after drinking | Oxygen dips stayed raised, with no more alcohol | 20 men with no symptoms, 1981 |
| Sleeping on your back | Two in three were worse on their back | 12,036 sleep-center patients with apnea |
When is the drop more than normal?
When it keeps coming back, night after night, and other signs come with it. The signs doctors ask about are loud snoring, someone seeing you stop breathing in your sleep, feeling sleepy in the day even after a full night, and high blood pressure. If someone has seen you stop breathing, or you snore loudly and you’re sleepy by day, ask your doctor about a sleep study, whatever your watch says (how to know).
Check your own number in the tool below. Pick “My overnight average” or “The lowest point last night”, and it will judge it against the right group.
8 sources
- Stradling JR, Chadwick GA and Frew AJ, Changes in ventilation and its components in normal subjects during sleep, Thorax, 1985pmc.ncbi.nlm.nih.gov
- Shapiro I and others, Pulse oximetry values from 33,080 participants in the Apple Heart and Movement Study, npj Digital Medicine, 2023pmc.ncbi.nlm.nih.gov
- Gries RE and Brooks LJ, Normal oxyhemoglobin saturation during sleep: how low does it go?, Chest, 1996europepmc.org
- Apple Support, About Blood Oxygen on Apple Watch, checked 2026-11-14support.apple.com
- Kolla BP and others, The impact of alcohol on breathing parameters during sleep: a systematic review and meta-analysis, Sleep Medicine Reviews, 2018europepmc.org
- Taasan VC and others, Alcohol increases sleep apnea and oxygen desaturation in asymptomatic men, American Journal of Medicine, 1981europepmc.org
- Rosa CFA and others, Analysis of positional obstructive sleep apnea features in 12,000 patients at a sleep center, Sleep and Breathing, 2026europepmc.org
- Chung F, Abdullah HR and Liao P, STOP-Bang questionnaire: a practical approach to screen for obstructive sleep apnea, Chest, 2016europepmc.org
