The Sleep Wave is a personalized heart rate pattern that shifts each night, showing how well your body has recovered. It’s not just cool to look at; it’s simple to understand, unique to you, and backed by real science.
So, humanity has taken centuries to reach its current state with sleep science, from ancient ideas to today’s tech-backed insights. Let’s take a quick look at how far we’ve come.
What happens when you sleep
Even though sleep feels mysterious — like we just switch off and have no clue what's happening — scientists actually know a lot about it. Your body goes into «repair mode».
Here’s the breakdown:
Breathing slows down during deep sleep, then picks up and can become irregular in REM sleep.
Heart rate also slows to its lowest in deep sleep, but jumps back up during REM to about the same as when you're awake.
Muscles relax gradually, and your energy expenditure drops. You’re mostly paralyzed by REM to keep you from acting out dreams, except for eye muscles (hence, rapid eye movement).
The brain flushes out unnecessary junk and resets key chemicals, helping you feel sharp the next day.
Dreams happen in REM and can be vivid or strange, but they can occur at any stage.
Hormones like melatonin and growth hormone get produced, helping you manage stress and repair tissues.
The immune system gets kicked into high gear, helping you fight off infections and recover faster.
All this happens while your heart rate, blood pressure, and body temperature drop and your muscles fully relax. If anything disrupts these conditions, your body’s recovery process doesn’t work as well. Since it’s tough to measure all of this directly, we use heart rate as a key indicator of recovery. To make it easy to understand, we created the sleep wave metaphor.
What is your Sleep Wave, and how it works
Your Sleep Wave is a visual summary of your heart rate pattern during sleep, showing how well you recovered overnight. It was born from a simple yet ambitious goal: to create the best sleep analysis on the market — scientific, easy to understand, educational, and visually stunning. While others rely on generic circles and charts, we wanted something unique that truly captures each person’s sleep story. When we started testing the Sleep Wave algorithm, it became clear that the wave was the perfect fit, meeting every standard we set and revealing a comprehensive picture of sleep over time.
From a tech and science angle, the Sleep Wave needed to be developed based on heart rate. Heart rate is the only objective metric we can get from a wearable while you sleep, and it holds surprising depth. Rather than just giving you an average, we turn your heart rate into smooth, individualized patterns that mirror circadian rhythms. By comparing the start, middle, and end of your sleep to your personal baseline, the Sleep Wave gives you insights into how well you rested in a way that’s uniquely yours.
How it works:
We start by figuring out your baseline pattern using your heartbeat from your best recovery nights. We need at least 30 nights of sleep data from your Apple Watch to get this. Even if your sleep isn’t great overall, it helps compare how one night stacks up against your usual pattern. As your sleep habits improve, your baseline will update too.
Next, we analyze your heart rate from last night and create a wave. The wave gets its shape by smoothing out those heart rate readings.
We then compare last night’s wave to your usual pattern and let you know how well your body recovered.
There’s some solid science behind it, so let’s dive into what those patterns mean.
The science behind it
If we put it simply, science shows that when you fall asleep, your heart rate gradually drops, hitting its lowest point around the middle of the night — often syncing up with peak melatonin levels and one of the deep sleep stages (which vary in depth, by the way) to help your body recover. Toward morning, it slowly starts to rise, prompted by a natural boost in cortisol as your body prepares to wake up. This creates a sleep pattern that can vary from a smooth arc to a nearly flat line, depending on your body's needs that night.
Why your sleep pattern matters:
It’s a reflection of your sleep quality
If your heart rate stays elevated during sleep, it’s usually a sign of poor sleep quality, often due to factors that keep your body from fully relaxing. Studies link higher nighttime heart rates with a greater risk of heart issues, which is why that gentle dip in heart rate is so essential.It points out potential sleep disruptors
External factors like alcohol or evening workouts can impact your sleep pattern. Alcohol disrupts your sleep cycle, raises your heart rate, and gets in the way of full recovery. Exercise, on the other hand, can be beneficial if timed right.It helps you see if you’re getting enough sleep
For example, a likely sign that you've actually slept enough is when your typical wake-up pattern kicks in right before you wake up. So, let’s say your heart rate usually picks up a bit before waking up, but this time it didn’t, and your sleep wrapped up on a kind of flat, low phase — that’s a good indicator you probably didn’t get enough rest.
So, your Sleep Wave is more than just a nice-looking curve — it’s a crucial sign of how well your body resets each night, shaped by your habits, health, and lifestyle, and backed by science.
Demystifying your Sleep Wave
Here’s what guarantees you a green wave and solid recovery:
Your heart rate gradually lowers as you fall asleep.
It dips around the middle of the night.
It rises gradually before you wake up.
Hit all three, and you’ll probably get a green wave and wake up feeling amazing. If you meet two, you’ll likely see a yellow wave and feel fine. If you miss all three, expect a red wave and a rough morning. For more details, keep reading.
Every part of your wave — like the color, height, and shape — tells a story.
The color
Your wave comes in 4 main colors:
Blue means your heart rate was lower than usual, which can be a good sign. A lower heart rate often indicates a well-conditioned heart, like in athletes, but it's not always positive.
Bradycardia — a heart rate below 50-60 bpm — is more common in athletes and can also be linked to health issues like heart disease, Lyme disease, or sleep apnea. Certain medications, substances, and conditions like anorexia or hypothyroidism may also lower heart rate.
Fear-induced bradycardia, sometimes triggered by anxiety or nightmares, can also drop heart rates to around 40 bpm during sleep, often in response to emotional stress or disrupted sleep patterns. So, if your typical sleep heart rate is 60 bpm and last night it dropped to 57, that’s usually a good sign. But if it suddenly dips to 47, that’s a «blue» flag worth looking into.
Green means your heart rate stayed within your normal range, so your sleep quality was about the same as usual.
Yellow reflects a slightly elevated heart rate, meaning your body was likely dealing with something — exhaustion, alcohol, recovery from a tough workout, or fighting off illness.
Red means your heart rate was much higher than normal, which is usually a sign that your body went through something intense, like illness, heavy stress, or an intense workout.
Your wave also shows gradients and dots. Gradients indicate where and when one color transitions to another, while the dots mark the heart rate readings your watch captured throughout the night.
The рeight
The height of your wave shows where your sleep is compared to your baseline. In general, if it’s higher, you probably slept worse. If it’s lower, it’s a sign of better sleep. Note that there are exceptions, especially for certain health or sleep conditions that affect your heart rate.
The shape
The shape is the most fascinating part of your wave. Your wave has a beginning, middle, and end, and science shows there are three key things to focus on:
A downward trend after falling asleep. This shows your body went into repair mode and started recovering. If your heart rate increases instead, it could mean exhaustion or something out of your control, like snoring or sleep apnea.
An upward trend before waking up. This usually happens because your sympathetic nervous system kicks in, causing an increased heart rate as your body prepares to wake. This is normal and helps ensure enough blood flow and oxygen for being awake. However, researchers think this morning surge, especially during REM sleep, might explain why vulnerable people often experience heart attacks in the early hours.
The minimum heart rate in the middle of the night. A lower minimum heart rate during this period indicates good recovery from daily stress and exertion, indicating a well-functioning autonomic nervous system with strong parasympathetic activity.
Why we all should stop chasing the hammock
The hammock-shaped heart rate curve is often romanticized as a marker of ideal recovery. Here’s the hard truth: that shape alone tells you very little about your actual recovery.
While it’s true that a dip in heart rate during sleep can reflect parasympathetic nervous system dominance (your body’s «rest and digest» mode), the pattern itself isn’t inherently beneficial. It's just how your heart reacts to various stages of sleep — particularly deep and REM stages. What really matters is how low your heart rate drops during that dip, how high it rises, and when these shifts occur.
If you want to understand your sleep, focus on more meaningful, science-backed things like a downward trend after falling asleep, an upward trend before waking up, and a heart rate dip somewhere in the middle of the night. The shape may differ from the hammock, but you’ll still get the best sleep.
Some real-life examples
Thus, the color, the height, and the shape of your sleep wave tell you a complete story of your nightly recovery. Let’s take examples.
The sky-high wave
Here’s how a sleep wave usually looks when people overdo it — whether it’s drinking, stressing, or over-exercising. It’s all red, spikes high, has a strange shape, and basically shows zero recovery. The coolest part is that literally seeing what you're doing to yourself can really push you to improve your habits.
How to avoid it:
Stick to your sleep schedule and aim to be in bed within your bedtime window.
Skip alcohol at least 3 hours before sleep, and avoid coffee or other stimulants 5–6 hours before.
Keep your room quiet, dark, and cool, and stick to basic sleep hygiene.
The rebellious wave
There is no dip, no gradual lowering, and an excessively elevated heart rate at the end, which results in poor recovery and a brutal wake-up.
How to avoid it:
Remind your brain that nightmares are officially off-limits;
Try some relaxing sleep stories to set yourself up for sweeter dreams.
Wave, interrupted
This wave is all about what’s happening at the end. Even though it’s blue, with a nice dip in heart rate around the middle of the night, it didn’t fully return to normal by the time I woke up. That lower heart rate shows recovery is in progress, but if it doesn’t rise high enough by morning, it suggests recovery wasn’t fully complete. So, while it doesn’t mean your sleep was bad, it does mean your body didn’t get all the rest it needed. This wave’s a classic for new parents — those middle-of-the-night wake-ups leave their recovery hanging. The same goes for anyone jolted awake by an alarm or that one neighbor who thinks 6 AM is the perfect time to mow the lawn.
How to avoid it:
Stick to your sleep schedule and aim for enough hours.
If you see this wave, give your body some extra grace during the day — think of naps and restful breaks.
The groggy wave
This wave dips just before wake-up, which pretty much guarantees you’ll wake up feeling groggy. Add in some sleep inertia, and you’re setting yourself up for a rough morning.
How to avoid it:
Stick to a regular sleep schedule and make sure you’re clocking enough hours.
If you see this wave, try some wake-up hacks, such as energizing breathwork, splashing cold water on your face, or soaking up some bright light.
Why the Sleep Wave is game-changing
It’s unique, easy to understand, and backed by solid science. Here’s why it stands out:
Simple, beautiful, and never the same. The Sleep Wave is a powerful visual tool with a clean design that changes every night, keeping you curious, engaged, and in tune with your sleep patterns.
Insightful, accurate, and personal. With just your heartbeat, we’re breaking new ground. Instead of using someone else’s formula, we rely on our own algorithms — years in the making and proven across multiple fields. This means you can actually see how a stressful day or a nightcap impacts your wave, gently nudging you toward habits that benefit your sleep and overall wellness.
Truly personalized and backed by science. We dive deep into your Apple Watch data to demystify what’s happening in your body while you sleep, analyzing heart rate patterns on nights you’ve slept well to create a one-of-a-kind wave that’s uniquely yours each night. Unlike others, we don’t rely on generic age ranges or anecdotal evidence — we use hard science to give you insights based on your unique «normal», not someone else’s. And that’s a big deal.
In the end, your Sleep Wave isn’t just data — it’s the story of your night, a reflection on your habits, and a map to better rest.
You’re not just tracking sleep — you’re discovering what actually happens while you sleep and what truly fuels your body. Every heartbeat wave, every pattern, every shift brings you closer to understanding what really makes for a restful night and a recharged day. This isn’t sleep science you have to decipher — it’s sleep science working for you. So, take it all in, tweak what you need, and wake up to the power of knowing exactly what your body needs to thrive.