Why sleep quality is so important – and so difficult to measure

How did you sleep last night? Your response might depend on how long you were in bed, how much of that time you spent tossing and turning or whether you feel rested. But it might also depend on whether you exercised today, what your wearable device says, or when you are being asked.

“Everyone has their own definition of sleep quality – and that is the problem,” says sleep researcher Nicole Tang at the University of Warwick, UK.

Though sleep quality and what defines it is a puzzle scientists are still figuring out, we do know that a good night’s rest involves a series of sleep cycles, the distinct succession of phases of brain activity we experience during sleep (see diagram below). And for most of us, each stage of those cycles is necessary to wake up feeling refreshed. The average person experiences four to five complete cycles during a night and disrupting these can come with health consequences, both in the short and long term.

“Poor sleep quality is associated with many adverse physical health outcomes,” says Jean-Philippe Chaput at the University of Ottawa, Canada. Similar to what you can expect from not sleeping enough (see “Why your chronotype is key to figuring out how much sleep you need”), these include a higher risk of cardiovascular disease, stroke, hypertension, type 2 diabetes and weight gain.

Although there is no definitive consensus on what defines sleep quality, researchers and doctors frequently analyse sleep with an electroencephalogram (EEG), which tracks brain activity during sleep…