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.
This article is part of special series investigating key questions about sleep. Read more here.
“Everyone has their own definition of sleep quality – and that is the problem,” says sleep researcher at the University of Warwick, UK.
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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 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 , , , and .
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 cycles, or using actigraphy, where body movement is monitored throughout the night as a measure of wakefulness. Such measurements show that the factors with the greatest impact on what include how long it takes to doze off, how often you wake up and sleep efficiency – the percentage of time in bed that is actually spent in slumber. “Usually, the case is that not just one parameter predicts sleep quality – it’s a bunch of different parameters added up together,” says Tang.
But how those parameters stack up doesn’t always tally with subjective experience. For example, grouped them by sleep quality using EEG measurements, finding that poor sleepers spent less time in the deeper phase of non-rapid eye movement (NREM) sleep compared with better sleepers. However, self-reported measures of sleep quality didn’t match the EEG-based ones. from more than 250 people over seven nights found that subjective sleep ratings were only moderately related to objective metrics, with sleep efficiency the most important variable in determining whether participants reported better-quality sleep.
What that means is that your EEG or actigraphy measurement can reflect what looks like a stellar night’s rest, and yet you would still rate your sleep quality as poor, and vice versa. Exactly why that can be the case isn’t entirely clear, but other research backs up the idea that what happens in bed isn’t the only way we determine the quality of the sleep we have had. In a 2022 study, Tang and her colleagues found that participants’ perception of how they slept was , such as their current mood or their level of physical activity. “What you do during the day could affect your evaluation of the sleep the night before,” says Tang.
This frustrating situation led a panel of sleep experts who of the “elusive, amorphous, and multi-dimensional construct of sleep quality” to conclude that “ultimately, the determination of ‘quality sleep’ remains largely subjective and inconsistently quantifiable by current measures”.
That sleep quality is so difficult to assess objectively should give us pause when considering data from wearables that provide a sleep quality score. Many of these are based on measurements, such as heart rate or movement that can accurately determine , but the makers of these gadgets typically don’t explain how these factors are weighted to determine the final output. Some experts caution against giving too much importance to these scores, as they can be unreliable and increase anxiety around sleep.
Even if we can’t always accurately assess our sleep quality, there are things we can do to attempt to get a better night’s sleep – for example, not drinking. Alcohol may help you nod off and increase the amount of deep NREM sleep in the first half of the night, but in the second half and impairs rapid eye movement sleep. Maintaining a regular sleep schedule and good sleep hygiene habits will also help.
Of course, some sleep fragmentation is unavoidable – tending to a crying baby, nighttime visits to the toilet – and circumstances change over your lifetime. So avoid fretting about one night’s interrupted sleep: precisely because sleep quality is so subjective, if you start feeling anxious about it, you may wake up thinking your night went even worse than it did.
