Compass is still being developed and is effectively in beta. Some content and features may be incomplete, change, or not work as expected.

Content is anchored to trusted sources where available, but Compass has not yet been clinically reviewed. It provides general information rather than medical advice.

Feedback helps us identify gaps, errors, usability problems and opportunities to improve Compass over time.

How Compass is developedGive feedback

Tests and measurements

Overnight oximetry

A simple overnight recording of oxygen levels from a probe worn on a finger.

What it means

A clip on a fingertip records oxygen levels through the night. It is far simpler than a full sleep study and is often done first, though it shows less: oxygen can look acceptable while carbon dioxide is already building up.

Why you might hear it

  • It is often the first overnight test offered, at home.
  • A normal result does not always settle the question, which is why more may follow.

People who may help

Where Compass goes into this

About this explanation

These are plain explanations of words you may come across, not advice about your own situation. Where a word matters to a decision, the question pages go into it properly, and your own team is the place to take it.