One article of sleep science, included because everything else in this series stands on it.
| Stage | What it's doing | What it feels like when you're short |
|---|---|---|
| Deep sleep (slow-wave) | Repairing you physically — muscle and tissue. Releasing growth hormone. Clearing out the day's metabolic waste. Filing what you learned into long-term memory. Keeping your immune system going. | Tired in a way sleep doesn't seem to fix. Workouts you don't bounce back from. Reaching for words, losing names. Catching whatever's going around. |
| REM sleep | Working through what happened and how you felt about it. Locking in skills and learning. Most of your dreaming. | A shorter fuse than usual. Small things landing harder than they should. New learnings taking longer to stick. Feeling irritable without knowing why. |
Your night isn't one even stretch of uninterrupted sleep. Your sleep runs in cycles of roughly ninety minutes, and the mix changes as the night goes on. Deep sleep is front-loaded — most of it is over before you're halfway through your night's sleep. REM is back-loaded — your longest run of it arrives in the final hours before you wake.
You can feel this distribution at work when a night gets cut short — an early alarm to catch a flight for vacation, for example. If you shorten your sleep by two hours, you don't lose a quarter of everything — you mostly lose REM, which is why a short night leaves you emotionally brittle and irritable rather than just sleepy. And anything that disturbs the second half of the night, like a 3 a.m. wake-up as a late-night drink works its way out of your system, takes REM, not sleep in general.
And if you don't want to rely only on how you feel or on a sleep journal to figure out your sleep stages, most wearables — Whoop, Garmin, Apple Watch, Oura and others — report your sleep stages and the time you spend in each on a nightly basis. They aren't the precise instruments you'd find in a sleep clinic, and any single night's numbers may vary, so don't read too much into one poor “sleep score.” If you have a wearable, watch your trends, cast them against how you feel, and note the effect a late-afternoon coffee or a nightcap has on the numbers and on how you feel. A wearable is one of the best ways to stop guessing about your sleep, and it will make the impact of everything you learn in the next articles easier to see.
There's also this, and if you're past forty it's important to know: deep sleep declines with age. By your forties and fifties you're getting less of it than you did at twenty. It's expected, it's not your imagination, and it's why “just get eight hours” may not be 100% useful advice in midlife. It's also why the right question was never “how long did I sleep?” but “how much of it was deep and REM sleep?”
And it's why we care about restorative sleep rather than hours in bed. Deep sleep is when your brain does its housekeeping, and researchers are taking the link between years of poor deep sleep and long-term brain health increasingly seriously. That's a live question, not a settled one — but it's the reason we measure what we measure, and the standard we hold our own product to.
So that's what you're protecting: the front of the night for deep sleep, the back of the night for REM. The next article covers how to protect them — eight things you control, starting with the one that does more work than the other seven put together — and it's probably not the one you'd guess.