Every morning millions of us wake up and, before we’ve even decided how we actually feel, consult a tiny piece of technology to find out how we slept.
Apparently I slept for seven hours and 42 minutes, was awake for 37 minutes, spent an hour and 19 minutes in REM, got 48 minutes of deep sleep and earned a sleep score of 76. Which feels suspiciously like a C. Frankly, I don’t appreciate being graded before I’ve even had coffee.
Our watches and rings are giving us more information about our bodies than we’ve ever had access to before. Heart rate, heart rate variability, respiratory rate, blood oxygen, sleep duration, sleep efficiency and a colorful little graph showing exactly how much time we supposedly spent in light, deep and REM sleep. It’s fascinating information. There is just one small problem. Most of us have absolutely no idea what any of it means.
So, before you decide that your 42 minutes of deep sleep means you have six months left to live, let’s talk about what is actually happening after you close your eyes.
Sleep isn’t one long, unconscious event. Throughout the night, your brain cycles through two basic types of sleep: non-REM sleep and REM sleep. Non-REM sleep is divided into three stages, creatively named N1, N2 and N3. We move through these stages multiple times throughout the night, with a complete sleep cycle generally taking somewhere around 80 to 100 minutes.
N1 is the lightest stage and basically the waiting room between awake and asleep. Your muscles begin to relax, your breathing and heart rate start slowing and you’re relatively easy to wake. You don’t spend a tremendous amount of the night here, but you have to pass through it to get to the good stuff.
N2 is still considered lighter sleep, but you’re officially asleep now. Your body temperature drops, your muscles relax further and your brain activity changes. We actually spend a pretty significant portion of the night in this stage, so don’t look at a giant chunk of “light sleep” on your watch and assume you failed sleeping. Light sleep isn’t junk sleep. It is a normal and necessary part of the process.
Then we get to N3, the coveted deep sleep your wearable has probably convinced you that you desperately need more of. This is slow wave sleep, and it’s when your body gets some serious restoration work done. Heart rate and breathing slow, muscles relax, energy is restored and processes involved in tissue growth and repair take place. Deep sleep tends to happen more during the first part of the night, which is important to know when you’re staring at that graph wondering why you didn’t get equal amounts of it all night long.
Then there is REM, or rapid eye movement sleep. This is where things get weird. Your brain becomes highly active, your eyes move rapidly and vivid dreaming commonly occurs while most of your major muscles are temporarily kept from moving. REM is particularly important for processes involving learning, memory and mood, and unlike deep sleep, REM periods tend to get longer as the night progresses. This is one of the many reasons chopping two hours off the end of your sleep isn’t necessarily the same thing as simply losing two random hours.
Now that you understand the colorful graph, we need to have a conversation about the device producing it.
Your watch does not actually know when you’re in REM sleep.
I realize this may feel like betrayal.
In an actual sleep study, sleep stages are determined using measurements that include brain activity and eye movements. Your watch or ring isn’t reading your brain waves while you sleep. Instead, consumer devices typically use things they can measure, including movement and heart rate related information, and then an algorithm makes its best estimate about what’s happening.
And to be fair, they’re getting pretty impressive. Research comparing popular consumer wearables with laboratory sleep studies has found that many are quite good at determining whether someone is asleep or awake. Sleep stage accuracy, however, is more variable. That means I would pay far more attention to patterns over time than I would one random Tuesday when your watch announces that you received 31 minutes of deep sleep and apparently should spend the day reconsidering all of your life choices.
This is where I think health technology can either be incredibly useful or make us completely insane.
If your wearable shows that you’re consistently getting six hours of sleep when you swear you’re getting eight, that’s worth paying attention to. If you notice your sleep quality tanks every time you drink alcohol, eat a huge meal late at night or stay up scrolling until midnight, congratulations: your expensive piece of technology has provided you with information you can actually use.
But if you wake up feeling rested, you’re functioning well during the day and your watch gives you a sleep score of 71, please don’t immediately decide you’re terrible at sleeping. The goal was never to win sleep.
Instead, look at the bigger picture. Are you giving yourself enough opportunity to sleep? Are your bedtime and wake time somewhat consistent? Are you waking repeatedly throughout the night? Are you exhausted during the day? Do you snore heavily or wake up gasping? Has something noticeably changed from your normal pattern? Those questions matter more than obsessing over whether you achieved the exact amount of REM your app thinks you should have gotten.
I love health data. Give me the graphs, the numbers, the trends and all the nerdy little metrics. But data is supposed to help us understand our bodies, not make us afraid of them.
So tomorrow morning, before you let your wrist tell you whether you had a good night’s sleep, give yourself about five seconds.
How do you actually feel?
Then you can check your score. Preferably after water and coffee.
