Many people aren't sleeping nearly as much as they think they are. They may be lying in bed for 7 hours, but are only actually sleeping for 5 or 6. And, not surprisingly, they feel tired the next day.

Experts refer to this concept as "sleep efficiency" — the percentage of time lying down that you're actually sleeping. Going to bed at a reasonable hour doesn't matter as much if you're sleeping inefficiently. Luckily, getting a sense of your sleep efficiency is getting easier and easier, particularly with lifestyle trackers such as Fitbit. And there are lots of tips for improving your sleep efficiency, too.

The science of sleep efficiency is still young. There's no magic number for efficiency that's been proven as linked to terrible health — but it can still be a handy metric for many people to get a sense of what's going on at night. (Ultimately, what really matters is how well-rested and attentive you feel during the day, but that's a complex thing that can't be measured with one number.)

How to measure your sleep efficiency

There are basically three factors that make up good sleep: how long you sleep, when in the day you get that sleep, and the efficiency or quality of that sleep.

Until inexpensive fitness trackers started hitting the market, sleep efficiency was an obscure metric that generally only sleep doctors had heard about. Sometimes researchers would measure patients' sleep efficiency using actigraphy, which involves an accelerometer strapped to the wrist.

TODAY, many fitness trackers CLAIM TO measure sleep efficiency

But nowadays, many fitness trackers use accelerometers that detect motion to calculate how much you've moved in a given day. They can also double as sleep monitors by distinguishing sleeping from unrestful tossing and turning. Several smartphone apps, such as the Sleep Time app, also track sleep by using the phone's internal accelerometer. This requires leaving the phone on your bed throughout the night. (Most of them also claim to track lighter and deeper sleep in order to wake you up with an alarm at a non-groggy time.)

Many of these devices or apps will spit out a number for sleep efficiency — dividing the amount of time that the device says you slept by the time you know that you spent in bed. Some, like the Fitbit, will remove the time before you fell asleep from the calculation.

What's the ideal sleep-efficiency number?



As far as the science is concerned, there's no particular number for sleep efficiency that's considered optimal. There just hasn't been enough research yet to determine something that specific. However, some experts estimate a rough ballpark of 85 percent or above as a decent place to be.

Rachel Salas, a neurologist who focuses on sleep at Johns Hopkins, puts it this way, "I wouldn’t necessarily seek help if you’re looking at sleep efficiency based on a device and see no other signs [of a problem]." Those signs could be things like excessive, unexplained sleepiness during the day or trouble falling or staying asleep at night.

So why even measure sleep efficiency? In part, because people could find it helpful to track sleep efficiency over time — and see how it differs in response to changes in their own lives: "The value of that number is in whether it changes," says Michael Twery, director of the National Center on Sleep Disorders Research at the National Institutes of Health. (And also, who wants to waste time in bed not sleeping?)

How to sleep better and more efficiently

Salas offers several tips for improving sleep efficiency:

1) Turn off TVs, computers, and smartphones an hour before bed. Salas recommends shutting all your glowing screens off an hour before bed — or even 30 minutes before, if you simply can't handle an hour. (The reason behind this is that blue light is a key signal that tells your brain that it's daytime. One possible cheat is the app F.lux, which makes computer screens less blue as the day goes on. Although there are no scientific studies on its validity, it can't hurt to try, and it has many devoted users in the Vox office.)

2) Keep a consistent sleep schedule. Go to bed and wake up at the same time, or relatively the same time, every day. When you don't do this, you essentially put your body into self-imposed social jetlag, which throws you off kilter.

3) Don't exercise intensely right before bed. You might think that physically exhausting your body will help you rest, but it can actually raise your body temperature and make it more difficult to fall asleep. (Your internal body temperature is intimately related to your sleep cycle.)

4) No booze right before bed. No nightcaps, by which I mean alcoholic beverages and not hats. Although such drinks might make it easier to pass out in the first place, they also mess up your sleep and can make your nights more restless, overall.

Note that you don't have to have 100 percent efficiency to be a good sleeper. It's considered perfectly healthy to take up to 30 minutes to fall asleep at night after turning off the light. (In fact, if you regularly falling asleep within five minutes, "you probably have severe sleep deprivation, possibly even a sleep disorder," according to the National Institute of Neurological Disorders and Stroke.)