Most members arrive with months of data from a ring or a watch. The data is useful. It is also routinely read as more than it is.

What a wearable measures

Consumer devices measure movement, heart rate, heart rate variability and sometimes skin temperature. From those they estimate when you fell asleep, when you woke and how long you spent in each stage. Time asleep is usually within 20 to 30 minutes of a lab recording. Stage estimates are much weaker: agreement with a laboratory study for deep sleep is often around 60 percent.

Duration is measured. Stages are guessed.

What quality actually means

Sleep quality is not a stage percentage. Clinically, it shows up as how quickly you fall asleep, how often you wake, whether breathing stays regular and how you function the next day. A score of 82 says nothing about any of these on its own.

The most common thing we find behind poor sleep in members over forty is not a lack of deep sleep. It is disturbed breathing. Roughly one man in four and one woman in ten in this age group has moderate sleep apnoea, and most do not know it.

Signs that need a proper study

• Loud snoring, or a partner who has noticed pauses in breathing.
• Waking unrefreshed after seven hours or more.
• A heart rate that rises overnight instead of falling.
• Morning headaches, or blood pressure that resists treatment.

How we measure it

When the history suggests a breathing problem, we send a home sleep study: a small device worn for two nights that records airflow, oxygen and breathing effort. It gives an apnoea hypopnoea index, the number of breathing events per hour. Below 5 is normal. Above 15 is moderate and usually needs treatment.

Member 0344 slept 7 h 40 min on average and had a ring score in the eighties. His study showed 23 events per hour and oxygen dropping to 84 percent. Six weeks after starting treatment his morning blood pressure had fallen by 9 points. The ring score barely changed.