Sleep, in Four Acts
What deep sleep, REM, and everything in between are doing while you’re unconscious
Sleep is one of those topics where, if it isn’t giving you trouble, you don’t give it much thought. I don’t track mine. I go down early, I sleep through the night, I wake up rested, and I consider myself genuinely lucky on that front.
Sleep is also one of the most subjective experiences we have, and modern life has wrapped it in metrics. Oura rings, Whoop straps, Apple Watches, Eight Sleep mattresses; we are now hyper aware of numbers that no previous generation ever saw. Maybe that’s a good thing, if it prompts more people to focus their rest…or maybe it’s creating a new kind of low-grade anxiety that kicks in the moment your head hits the pillow.
Here’s something I’ve noticed: the conversation about sleep rarely starts as a conversation about sleep. It starts as someone asking if they should drink more coffee. It starts as a mid-afternoon crash, and what to do about it. It starts as a general draggy feeling, and what supplement might help. It starts as brain fog, mood, or a memory that feels foggier than it used to.
Almost every time, sleep (or more specifically, a lack of it) lies at or near the root of the issue.
Sleep itself is genuinely fascinating. The questions underneath it are some of the most interesting in biology: why we power down at all, what happens while we’re unconscious, why each phase is timed so precisely that if you miss it you can’t get it back the same way, whether you can ever truly recover from a sleep debt once it’s built up, why we dream…the list goes on.
I won’t answer all of it today. Some of it, particularly how your brain cleans itself during deep sleep, deserves its own post.
For today, let’s start with the four stages of sleep, what each one does, and how to use your sleep tracker without letting a bad score ruin a perfectly good morning.
Sleep is not one thing
When you fall asleep, you don’t just drop into “asleep” and stay there. You cycle through four distinct stages, roughly every 90 minutes, four to six times a night. Each stage does something completely different. Each one is trying to fix a different part of you.
Miss too much of one, and you can sleep eight hours and still feel terrible. Get the right ratio, and you can sleep seven and feel like a different person.
First, a quick note on actual sleep tracking
Sleep specialists use something called an electroencephalogram (EEG) to measure the electrical activity of your brain. It works by placing small electrode sensors all over your scalp. You look like a mummy from the neck up.
Your neurons (brain cells) communicate by firing tiny electrical signals, and when millions of them fire in sync, that activity is strong enough to be detected through the skull and recorded as a wavy line on a graph. Those waves are what sleep researchers read to identify which stage you’re in.
Each stage has a signature pattern:
Alert wakefulness produces fast, small beta waves
Drifting off (N1) shows slower theta waves
N2 shows the sleep spindles and K-complexes I wrote about
Deep sleep (N3) produces the big, slow delta waves
REM looks almost identical to being awake on an EEG; which is part of what makes it so strange
EEGs are the gold standard for sleep science. When you see a sleep study done at a clinic, that’s what’s happening: electrodes glued to the scalp, recording your brain’s electrical activity all night.
Consumer wearables (Oura, Whoop, Apple Watch) don’t do EEG; they estimate your sleep stages using indirect signals like heart rate variability, movement, and skin temperature. It’s why their stage readings are directionally useful but not clinically precise, and we’ll talk more about that.
Now back to the main event
The four sleep stages are:
N1 (light sleep, the on-ramp)
N2 (light sleep, the bulk of the night)
N3 (deep sleep, also called slow-wave sleep)
REM (rapid eye movement, where dreams mostly live)
N1 and N2 are light sleep. N3 is deep sleep. REM is its own thing entirely. Let’s take them one at a time.
N1: The on-ramp
This is the brief window (usually one to seven minutes) where you’re drifting off. Your muscles start to relax, your heart rate and body temperature drop, and your brain waves transition from the alert alpha waves of wakefulness into slower theta waves. If someone nudges you during N1, you’ll swear you weren’t asleep yet; and you’d half be right, because N1 is the most unstable form of sleep you experience.
A few things worth knowing about N1:
This is where hypnic jerks happen, those sudden full-body twitches that make you feel like you’re falling. One leading theory is that your brain briefly misreads your rapidly relaxing muscles as falling, and sends a reflexive jolt to catch you. Harmless, but more common when you’re overtired, overcaffeinated, or stressed.
N1 acts as a kind of memory gate. Information you try to learn in the minutes right before sleep mostly doesn’t stick; your brain is already closing the intake doors. (This is why scrolling in bed is worse than just wasted time; you’re not actually retaining any of it.)
In plain English: N1 is the elevator doors closing. You only spend about 5% of the night here, but it’s the handoff that lets every other stage happen.
N2: The unglamorous majority
This is the stage that makes up roughly half of your total sleep. N2 is deeper than N1, much more stable, and far more active than it looks on a tracker chart. Your heart rate and body temperature drop further, and your brain starts producing two distinctive patterns found nowhere else in sleep: sleep spindles and K-complexes.
Sleep spindles are short bursts of high-frequency brain activity (11 to 16 Hz), each lasting half a second to a few seconds. They look, on an EEG, like little spindles of yarn. Their job is memory consolidation. During N2, spindles help transfer the day’s new information from the hippocampus (your brain’s short-term notepad) to the neocortex (your long-term filing cabinet). Facts, skills, muscle-memory movements; all of it gets copied over during spindle activity.
K-complexes are the other half of the story. They’re large, slow, isolated waves that appear either spontaneously or in response to a stimulus (a sound, a touch, a change in temperature). K-complexes serve as your brain’s night watchman: they evaluate incoming signals, decide whether they’re dangerous enough to wake you up, and if not, suppress them so you can stay asleep. When a dog barks outside and you sleep through it, that’s a K-complex doing its job. K-complexes also pair with sleep spindles to reinforce the memory-filing work; the K-complex creates a window of brain excitability, and the spindle rides inside it.
N2 is also where the majority of your sleep actually happens. You spend roughly half your total night here, across every cycle.
N3: Deep sleep, where the body rebuilds
This is the part that does the heavy lifting.
N3 is also called slow-wave sleep, because your brain is generating massive, synchronized delta waves: slow, powerful, low-frequency. Your heart rate hits its lowest point. Your body temperature drops even more. You are, in every meaningful sense, unconscious; waking someone up from deep sleep is hard, and if you manage it, they’ll be disoriented for several minutes.
Three things happen during N3 that are worth understanding:
1. Growth hormone release. The biggest surge of growth hormone for the entire day happens during your first deep sleep episode, usually within the first hour or two of sleep. Growth hormone isn’t just for teenagers growing taller; in adults, it regulates metabolism, tissue repair, and muscle recovery.
2. The glymphatic system turns on. Your brain doesn’t have a traditional lymphatic system like the rest of your body. Instead, it has the glymphatic system, which flushes metabolic waste (including beta-amyloid, the protein associated with Alzheimer’s) out of the brain. And it essentially shuts down during waking hours. The pulsing slow waves of deep sleep are what drive cerebrospinal fluid through the spaces between your brain cells, carrying the junk out.
3. Declarative memory consolidation. The facts and information you learned during the day get moved from short-term to long-term storage during N3. Students who cram until 2am and then get four hours of sleep are robbing themselves of the stage that would have actually cemented what they studied.
If you pay attention to anything in this post, this is it:
Why deep sleep comes first, and why your bedtime matters more than the hours you sleep
Two things decide how much deep sleep you get and the quality of it:
Sleep pressure: The chemical buildup (largely of a molecule called adenosine) that accumulates every hour you’re awake. By bedtime, it’s at its peak, and your brain discharges it by dropping hard into slow-wave sleep in the first two cycles of the night. Caffeine clears adenosine which is why it’s so effective in waking. you up.
Your body clock aka circadian rhythm: Your body is programmed to produce its deepest, most restorative slow-wave sleep during a specific window of biological night; for most adults, roughly 10pm to 2am. That’s when melatonin is highest, core body temperature is lowest, and your brain is most willing to throw off the big delta waves that do the actual repair work.
Here’s what I would like you to know: if you miss that window…that earlier-than-you-think window…that’s it, you miss it. If you go to bed at midnight, yes, you’ll still drop into deep sleep in your first few cycles, because sleep pressure is high. However, by that time, you’ve already missed part of the circadian window where deep sleep is biologically easiest to produce, and the slow-wave activity you do generate will be shallower than it would have been at 10pm.
You can’t add deep sleep back in the morning. You can’t nap your way out of it. You can’t sleep in on Saturday and recover what was missed on Tuesday. So, don’t miss it. I’m not even doing a “plain English” on this one; you get it. I promise you that going to bed earlier is life-changing.
When deep sleep is missed, it also causes the rest of your night to be pushed; your REM-heavy cycles (coming up next) are moved more into the early morning, right when your alarm is about to cut them short. The tracker might still show eight hours, but the architecture underneath is off.
REM: Where the brain rebuilds
Rapid Eye Movement (REM) sleep is arguably the weirdest stage. REM back-loads the night. You get most of it in the last third of your sleep, which is why cutting an hour off the morning hurts...that hour was almost entirely going to be REM.
Your brain during REM looks almost identical to your brain when you’re awake. Heart rate and breathing become irregular. Your eyes dart back and forth under closed lids (hence the name). And your body enters a state called REM atonia: your major muscles are temporarily paralyzed, presumably so you don’t act out your dreams.
Most dreaming happens here. But dreaming isn’t the point; it’s a side effect of what REM is actually doing.
1. Emotional processing. REM is where your brain processes emotional experiences from the day. The hippocampus, amygdala, and medial prefrontal cortex have a conversation during REM that doesn’t happen any other time, and that conversation is what lets you wake up and feel less raw about something that upset you yesterday. “Sleep on it” is really, get a bit of REM, and get back in business.
2. Memory consolidation, but different. Where N3 handles facts, REM handles emotional memories and pattern recognition. It’s the stage where the brain makes creative connections between things you learned days or weeks apart. The “I slept on it and figured it out” phenomenon is real, and it’s happening in REM.
3. Amygdala dampening. Studies show that REM-deprived people have heightened emotional reactivity the next day; things that would normally roll off you instead feel more dramatic. This is why you might get weepy, short-tempered, or reactive when you’re underslept…your emotional regulation system was shortchanged.
In plain English: Deep sleep repairs the body. REM repairs the psyche. Both are incredibly important to be a normal, functional human being.
The ideal ratio
In a healthy adult night, the rough breakdown looks something like this:
N1: 5%
N2: 45-55%
N3 (deep): 13-23%
REM: 20-25%
So on a good eight-hour night, you’re looking at roughly 90-120 minutes of deep sleep and 90-120 minutes of REM. Trackers vary wildly in their accuracy (none of them are doing actual brain wave measuring), but the trend data is more useful than the absolute numbers.
If your tracker shows you consistently getting under an hour of deep sleep, or under an hour of REM, take a look at that, and see where you might want to change your day to improve your night (so you can have an improved day!).
What tanks each stage
Alcohol is the villain of REM. It’s sedating, so yes, it gets you to sleep faster, but it suppresses REM for the first half of the night. Your brain then tries to catch up in the second half, causing REM rebound, fragmented sleep, and the 3am wake-ups you get after two glasses of wine.
Caffeine too late tanks deep sleep. Even caffeine consumed six hours before bed reduces slow-wave activity; you may fall asleep fine, but the deep sleep that should happen in the first few hours is shallower and shorter. Yes, that also goes for fast metabolizers…that part has nothing to do with the half-life of the molecule.
Late-night intense exercise can reduce deep sleep by keeping your core body temperature and cortisol elevated past the point where your body wants to drop into N3. Now, if 9pm is truly the only time you can exercise, don’t use that as an excuse to skip the workout…but if you have flexibility, try to exercise earlier in the day.
Stress and anxiety shorten REM and fragment the second half of the night. If you’re waking up at 4am with a racing mind, that’s your REM getting cut short. It’s counterintuitive, I know, but try to go into the night with the mindset (mantra?) that everything will be clearer in the morning, so your brain turns off. Basically…get out of your own way.
A warm bedroom. Deep sleep requires a core body temperature drop. If your room is over 68°F (20°C), you’re making it harder for yourself. This is also where a late afternoon/evening sauna, bath, or hot shower can help…again, counterintuitively, when your body heats you up, your body is forced to cool down, and that cooling drop will help.
What helps
For deep sleep:
Cool bedroom (65-68°F)
Magnesium glycinate or threonate at bedtime
Finishing heavy workouts before 7pm
Morning sunlight exposure within an hour of waking (sets circadian rhythm literally by getting light in your eyes)
Skipping the glass of wine with dinner
For REM:
Consistent wake time, even on weekends
Getting the full night (that last hour is almost pure REM)
Reducing alcohol
Managing stress before bed (breathwork, journaling, whatever works)
For both:
Go to bed at roughly the same time every night. Your body builds the architecture around consistency.
Don’t hit snooze. Each alarm fragments a sleep cycle and disrupts the REM you were in.
How to read your tracker without losing your mind
A few rules:
Watch the trend, don’t freak out about the night. A single bad score is inevitable; a week of scores tells you something.
Stages are estimates. Consumer wearables use heart rate variability and movement to guess your stage. They’re directionally useful, not clinically precise. If your Oura or EightSleep says 52 minutes of deep sleep, the real number could be 30, or it could be 75.
How you feel is still the source of truth. If your score was 92 and you feel like garbage, you feel like garbage. The data doesn’t override your experience; it contextualizes it.
Use it to find patterns. The goal isn’t a perfect night. It’s noticing that your deep sleep craters every time you drink wine, or that your REM spikes when you take magnesium, and adjusting accordingly.
In Case You Skimmed
Sleep has four stages: N1 and N2 (light), N3 (deep), and REM. Each does different work.
Deep sleep (N3) repairs the body: growth hormone release, tissue repair, and the glymphatic system flushing waste (including Alzheimer’s-linked proteins) out of your brain.
REM repairs the psyche: emotional processing, memory consolidation, and dampening the amygdala so you don’t wake up reactive.
Do not miss your deep sleep window. Deep sleep front-loads the night; REM back-loads it. Cutting an hour off either end costs you a specific stage because it is time-dependent.
Alcohol kills REM. Caffeine kills deep sleep. A hot bedroom kills both.
Your wearable is a trend tool, not a verdict. Use it to spot patterns, not to chase a score.
This week, try to put your phone down a little earlier and yourself to bed a little earlier…then see how you feel by next Monday.
See you then,
Susan
Longevity in the Wild
A friend in her forties told me she felt tired all the time. Sleep was fine, exercise was fine, bloodwork was “normal.” I looked at her recent bloodwork, eyeballing her ferritin specifically. Ferritin is the protein that stores iron in your body, and it's the most accurate window into whether you actually have enough. It’s particularly important in pre-menopausal women, who lose iron every month through menstruation.
For reference, the “normal” range runs 15 to 150 ng/mL; the optimal range is to 50 to 100 ng/mL. My friend’s level was 18. Technically inside the reference range, but functionally, splayed out on the floor. Needless to say, she’s following up with someone who understands the difference between normal and optimal. Moral of the story: symptoms also matter.