The guide

What changes your blood oxygen, and by how much?

By GyroscopeUpdated November 14, 202613 sources

Elevation, by a long way. Living a kilometre (about 3,300 feet) higher lowered a watch’s night number as much as about 27 years of getting older. Alcohol before bed comes next, then weight and age, which barely move it.

People tend to blame their age or their fitness for a low night. The bigger causes are usually where they slept and what they drank.

Why do people blame the wrong thing?

Because age and fitness are what we blame for everything else. Your knees, your sleep, your resting heart rate all change with age and training, so a low oxygen night sounds like more of the same.

The data says otherwise. In the 33,080-person Apple Watch study, the researchers modeled the night number against age, weight and elevation together. Elevation was far bigger than the others. A drink before bed isn’t in that model at all, and in trials it moved the lowest point of the night by more than 20 years of age moved the average.

A simple way to picture it

Think of your commute. Whether you leave at rush hour changes the trip far more than which lane you pick or which coffee you grab on the way. Fix the rush hour first, and the rest is detail.

For your blood oxygen, elevation is the rush hour, alcohol is a lane closure on some nights, and age and weight are the coffee.

How big is each one?

By day the same three model terms are a bit smaller, about 0.8 points per kilometre, 0.6 per 20 years and 0.5 per 10 BMI points.

Why not add them all up?

Each number is an average across a lot of people, from a different study. The elevation, age and weight terms come from one model of one watch model, the Series 6, and its straight line stops working above about 8,200 feet. The alcohol numbers come from 14 separate trials in 422 people, mostly men. Stacking them into one prediction for you would look precise and mean very little.

Your own nights also wobble. In the watch study, people’s usual night average sat within about 2.4 points either side of what the model predicted for someone like them, in about 95 of every 100 people if the spread is normal. That’s bigger than any single effect except elevation.

Why does elevation move it so much?

Each breath up high holds less air, so less oxygen. By the NASA standard atmosphere formula, a breath in Denver carries about 82% of the oxygen it does at sea level, and at 10,000 feet about 69%, the same figure the CDC gives. Your hemoglobin’s seats stay nearly full for a while, which is why the number only slips about a point by Denver. Higher up, the seats start emptying fast.

In numbers, the watch study’s night term was 1.17 points per kilometre and its age term 0.043 points per year. Divide one by the other and a kilometre higher comes to about 27 years of age. Oxygen by elevation works it out for any town, and the altitude section covers trips.

How does a drink lower tomorrow night too?

In 1981 a team led by Taasan gave 20 men with no sleep complaints a measured dose of alcohol, 2 ml for every kilogram of body weight, against a placebo night. Their oxygen dips nearly doubled, from 118 to 226 in a night, and their breathing pauses went from 20 to 110. On the next night, with no alcohol at all, the dips were still raised.

A 2018 analysis pooled 14 trials with 422 people. Alcohol lowered the night’s average oxygen by 0.6 points and its lowest point by 1.25, and added 2.33 breathing events an hour. In snorers it added 4.20, and in people with diagnosed sleep apnea 7.10. MedlinePlus advises cutting down or avoiding alcohol at bedtime if you snore.

If you track your heart too, the same drinks show up there. hrv.guide’s alcohol answer has those numbers.

Does sleeping position change your oxygen?

It can for people whose airway narrows in sleep. In 12,036 sleep-center patients with sleep apnea, 66% had their breathing problems mostly on their back, and 28% only on their back. Those patients had fewer breathing events overall and better oxygen than the rest. MedlinePlus’s advice for snorers is plain, don’t sleep flat on your back.

How much do weight and age move it?

Both a little, on their own. In the watch study, 10 points of BMI lowered the night average by about 0.6 points, and 20 years of age by about 0.9. In the 1996 sleep lab study of 350 healthy sleepers, people over 60 had a median of 95.1 through the night, and weight made no difference to their numbers.

Weight has a far bigger effect through sleep apnea. In the SURMOUNT-OSA trial, adults with obesity and moderate to severe apnea who took tirzepatide for a year had about 25 fewer breathing events an hour, against about 5 on placebo, and they lost more weight than the placebo group too. bodyfat.guide’s GLP-1 guide covers those medicines.

What changes the number without changing your oxygen?

A lot of what looks like a low night is the sensor. Cold skin, a loose band, some tattoos, dark nail polish on a finger device, and moving all change the number, according to Apple and MedlinePlus. Carbon monoxide does the opposite and can make the number look normal when it isn’t.

Skin tone changes the number too. In hospital patients whose finger oximeter showed 92 to 96, true oxygen was under 88 for 11.7% of Black patients’ measurements and 3.6% of White patients’. If your skin is darker, a number in the low to mid 90s may be flattering you. How far to trust it has the details.