The guide

What's normal while you sleep?

By GyroscopeUpdated November 14, 202610 sources

Your blood oxygen runs about a point lower asleep than awake, lowest around 1 am. The lowest point of a healthy night averaged about 90 in a sleep lab, so an 88 or 89 on your watch, once, is something healthy sleepers see too. A pattern over several nights is what tells you something.

Why does my watch show 88 at night?

Because you were asleep. Most people know the awake rule, 95 to 100, and then hold their sleeping number to it. An overnight 89 looks like an emergency next to a chart made for people sitting in a chair.

Apple says it on its own support page. “Slightly lower values while sleeping are expected, and some users might experience values below 95%.” The night is supposed to dip. The things to look at are how far it dips, how often, and whether it keeps happening.

Why does oxygen dip in sleep?

You breathe less. In a 1982 study of 19 healthy adults, the air they moved each minute fell about 6% in non-dreaming sleep and about 16% in dreaming (REM) sleep. In REM, each breath was about 73% the size of an awake breath. Less air in means a little less oxygen on the hemoglobin, so the number slips.

The body clock adds its own rhythm. Across 72 million measurements from 33,080 Apple Watch wearers, blood oxygen was lowest at about 1 am and highest at about 11 am. The study’s authors put the night dip down to the body’s own cycle of heart and lung activity across sleep and waking, more than to sleeping position.

A simple way to picture it

Think of a house thermostat set back at night. The heating eases off, the house drifts a degree or two cooler, and it warms up again in the morning. Nobody calls a repair service for that.

They’d call if the house got colder every night, or if the heating kept cutting out completely for a minute at a time. Those are the two things to look for in your nights, a drift that’s lower than your usual and repeated cut-outs.

How low does a healthy night go?

In 1996 two researchers, Gries and Brooks, asked that question in the title of their paper. They went back through 350 overnight sleep studies that came back normal, in people from 1 month to 85 years old, and looked at the oxygen numbers.

The median through the night was 96.5. The lowest point of the night averaged 90.4, with most people between about 84 and 97. People over 60 ran a little lower, with a median of 95.1. Sex, race and weight made no difference. People with obstructive sleep apnea in the same study ran lower.

That makes 90 a typical bottom for a healthy night, and a little under 90 common.

What’s normal for each night number?

The average band comes from Shapiro’s model of the Apple Watch study. For a 60-year-old with a BMI of 30 it’s about 92.4 to 97.2. For a 40-year-old in Denver, at 5,280 feet, it’s about 91.6 to 96.5. An average of 94 sits around the 5th percentile at sea level and right in the middle in Denver, so your Denver 94 is about a sea-level 96.

If your skin is darker, a watch or finger sensor can show higher than your true oxygen. 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’. That’s one more reason to judge a pattern of nights on one device, and to take snoring, pauses and daytime sleepiness seriously whatever the numbers look like.

Your number, explained puts any night number against people your age, at your elevation.

What makes one night lower than another?

• Where you slept. In the watch study, the night average fell about 1.2 points for every kilometre (3,300 feet) of elevation. Above about 9,000 feet, the CDC says some periodic breathing in sleep becomes nearly universal. Altitude has more.
• Alcohol. In 14 trials with 422 people, alcohol lowered the night average by 0.6 points and the lowest point by 1.25. In 20 men with no symptoms, given a measured dose, oxygen dips nearly doubled, from 118 to 226, and stayed raised the next night with no more alcohol.
• Sleeping on your back. In 12,036 sleep-center patients with sleep apnea, two in three had more breathing events on their back.
• Age and weight. Small. Twenty years of age lowered the night average by under 1 point, and 10 points of BMI by about half a point.
• The device. A loose band, cold wrists and a tattoo under the sensor can all change the number, Apple says.

How do you tell one low night from a pattern?

Run three nights the same way. Wear the same device the same way, sleep at the same elevation, and keep drinks the same or skip them. Each morning, note the average, the lowest point, any drinks, whether you slept on your back, whether anyone heard you snore or stop breathing, a morning headache and daytime sleepiness.

On the fourth morning you have either one low night or a pattern to bring to your doctor. If someone saw you stop breathing on any night, if the lowest point was under 90 on all three, or if the average was under 90 on two of them, that’s a pattern. The three-night check makes the dated card for you.

A sleep lab records your oxygen all night without a break, while a watch samples now and then when you’re still. That makes the lab’s lowest point and the watch’s lowest point two different measurements, which is one more reason the pattern counts for more than any single low.