How do you calculate Sleep Efficiency? It’s a simple but powerful metric–the percentage of the time you spend in bed that you are actually asleep. Sleep Efficiency is therefore a measure of sleep quality, and when considered alongside the rest of the WHOOP sleep pillar metrics, provides actionable insight into where your sleep could improve.
Looking at Sleep Efficiency alongside Sleep Performance provides a useful indicator as to where you can best target your sleep-improving efforts. This concept is broken down further:We’ll take a deep dive into Sleep Efficiency, share some original WHOOP research and demographic breakdowns, and also summarize what the world’s best academic sleep researchers have to say on the topic.
Let’s start off by answering a frequently asked question: What’s normal? We pulled the 1-million most recent sleeps recorded by our WHOOP users and plotted them below:
The graph above shows that the average Sleep Efficiency on WHOOP is 94.4%, meaning that for a typical 8-hour time in bed, the average WHOOP user is getting 7 hours and 33 minutes of actual sleep. This value is consistent with what academic sleep literature suggests given that the typical WHOOP user is young and fit, two characteristics which are known to be correlated with increased Sleep Efficiency. For a great review of the topic, check out this 2016 article by Drs. Michael Scullin and Donald Bliwise.
Since typical Sleep Efficiency varies quite a bit across demographics, we share some of the breakdowns:
Among WHOOP users, age is highly correlated with Sleep Efficiency, and displays an expected decline as users move from the youngest to oldest age groups. In fact, WHOOP users lose an average of about 1 percentage point of sleep efficiency per decade of added age.
Females on WHOOP sleep more efficiently than males on WHOOP in the youngest and oldest age groups, and sleep marginally less efficiently in their 20s, 30s, and 40s. Interestingly, female Sleep Efficiency is dramatically better than male Sleep Efficiency in our oldest age groups.
Day Strain and Sleep Efficiency have an interesting relationship, shown below, in which Sleep Efficiency peaks roughly around a Day Strain of 14 and declines for the lowest and highest Strains. For those readers not familiar with WHOOP, a Strain of 14 is roughly what we expect when a well-trained athlete runs 10k at a moderate effort.
Academic literature confirms that this pattern should be expected, as it’s been studied in both younger and older individuals. While most of the literature focuses on the differences in Sleep Efficiency following no-exercise vs light exercise days (the rise in Sleep Efficiency as Strain increases from 4 to 14), there is also literature supporting the decline in Sleep Efficiency seen when exercise intensity is greatest (the sharper fall as Strain increases beyond 18). One 2011 study demonstrated that following extreme exhaustive exercise, catecholamine (stress hormone) levels are increased and may remain increased for as many as two days, a change that could certainly impact sleep quality.
After looking at Sleep Efficiency and the previous day’s Day Strain, we next looked at how Sleep Efficiency varies among different types of athletes, specifically, by their primary sport (the activity they log most frequently on WHOOP). On average, soccer players are the most efficient sleepers and within our 15 most-popular sports, walkers and elliptical users are the least efficient.
While the below graph may suggest something interesting about the physiology of those who are best suited for one sport vs another, it is worth mentioning that the actual Sleep Efficiency experienced by a given user is more strongly determined by age, gender, frequency and intensity of exercise, and individual behavioral choices than it is by their primary sport.
Sleep Efficiency doesn’t just vary in expected ways, such as with age, but also shows some unexpected variation across geographic regions. We looked at the differences among our domestic and international users for our 12 most-represented countries.
We see in the graph above that European WHOOP users are generally sleeping more efficiently than are those of us in the United States and Canada.
Finally, in conducting the above analysis, we found that Sleep Efficiency, at least within the WHOOP user base, displays cyclical variation.
The graph above shows the WHOOP average daily Sleep Efficiency for the last 4 months. There’s a 7-day repeating pattern in which Sleep Efficiency is higher during the work weeks than on the weekends. This pattern can also be seen by binning the data by day of week (below). One thing we found amusing was how the cyclical pattern, which is very strong throughout October and November, breaks down around Thanksgiving and again during the holiday season in late December through New Years.
If we look at time in bed and hours of sleep (below), we see almost the opposite pattern from what we saw with Sleep Efficiency–greater sleep duration and time in bed are experienced on the weekend days. This likely explains the above trend in Sleep Efficiency because, as is demonstrated in a 1996 article published in Sleep, when users sleep late, the extended portion of their sleep tends to be less efficient than the earlier part of the night. It is therefore likely that the beginning of the users’ sleeps on weekends start out roughly equally efficient to any other night, but as they extend sleep past typical wake up times, Sleep Efficiency decreases sufficiently to bring down the overall average for the night.
Regardless of whether you are a young female soccer player with all the demographic advantages, or an older male whose primary activity is walking, the good news is that statistical predictors of Sleep Efficiency are only part of the story. The most significant determinant of it is your behavior. We suggest committing to some of the following changes to improve Sleep Efficiency:
We polled our 100 best sleepers on WHOOP for their tips and tricks for peak Sleep Performance, below are some of our favorite responses:
Check out your Sleep Efficiency trends by going to app.whoop.com.