Sense of Smell and REM Sleep in Sleep Apnea

Earlier studies found that people with sleep apnea often have a weaker sense of smell. Most of them looked at low oxygen and blocked nasal airflow as the likely link.

A 2026 study from China looked at sleep stages too. The clearest difference was in REM sleep, not in the lowest oxygen reading.

Who was in the study?

• 198 adults with obstructive sleep apnea, found on a full overnight sleep study at one hospital in Nanchong, China, between 2020 and 2024.
• 131 men and 67 women.
• People with nose problems, Parkinson's, Alzheimer's, a past head injury or recent steroid or antihistamine use were left out, since those can affect smell.
• Each did the Sniffin' Sticks test, scored 0 to 16 on each of three parts: how faint a smell they could pick up, telling smells apart and naming them, for a total out of 48.
• A score of 30 or under counted as a weaker sense of smell. 112 people scored there, and 86 above it.

What did it find?

The group with a weaker sense of smell, against the rest:

• Older: 56 against 40 years on average.
• More breathing events per hour: 28 against 18 on average.
• Less REM sleep: 9.6% of sleep against 15.5% (medians).
• Less deep sleep: 4.7% against 6.7% of sleep in stage N3 (medians).
• Less sleep overall: 356 against 412 minutes, and 74% against 88% of time in bed spent asleep.
• More men: 81 of the 112, against 50 of the 86.
• Fewer brief arousals: a median of 106 against 171 in non-REM sleep, and 13.5 against 22 in REM, counted across the night. The authors don't explain this one.

Smoking, alcohol and diabetes showed no statistically significant link with smell scores.

After adjusting for sex, three things stayed linked to a weaker sense of smell on their own: age, breathing events per hour and the share of sleep spent in REM.

What about oxygen?

The lowest SpO2 of the night was almost the same in both groups: 79.3% with weaker smell and 79.9% without, with no statistically significant difference. It showed no statistically significant link with smell scores either.

The authors note that an earlier study did link overnight oxygen below 90% to smell loss. They suggest the single lowest reading may miss the overall oxygen load across the night. This study didn't report time below 90% or ODI, so it can't say whether those would have shown a link.

For what those measures mean, see the oxygen desaturation index and what T90 means.

What can't it show?

• Cause. It's a one-time look back at records, so it shows what went together, not that less REM sleep weakens smell.
• Age. The weaker smell group was 15 years older on average, and smell fades with age. Age was in the same model as REM sleep, but a gap that big is hard to separate out.
• A comparison. There was no group without sleep apnea.
• Reach. One hospital in China, with more men than women.
• Nose airflow. The authors didn't measure it, and used only one smell test.
• Funding. A grant from the authors' own hospital, with no commercial ties declared.

The authors say a follow-up study with people starting CPAP is needed to see whether smell changes when sleep does.

What can you do with this?

A fingertip oximeter doesn't show sleep stages, so it can't tell you how much REM sleep you get. It does show your overnight oxygen, ODI and pulse, which you can track over time.

If you've noticed your sense of smell fading, mention it to your doctor along with how you sleep. For more on REM sleep and the brain, see REM sleep oxygen dips and memory.

Not a medical device. Does not diagnose, treat or prevent any condition. Discuss results and changes with a clinician.

References

1. Li Y, Huang R, Li X, Pan Q. REM sleep proportion: an independent predictor of olfactory dysfunction in obstructive sleep apnea hypopnea syndrome. Frontiers in Neurology. 2026;17:1815844. doi:10.3389/fneur.2026.1815844