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Deep Sleep: Why It’s Key To Your Health & How To Get More


If you’ve ever had even one night of lousy sleep, you know how critical catching Zzzs is to be able to function well the next day. After all, sleep is as critical to our survival as air, water, and food. And it helps keep everything from our brain to our immune system healthy. We spend nearly one-third of our lives snoozing, and yet many of us know very little about how we sleep best and why deep sleep is key.

Muse Deep Sleep Insights: Deep Sleep Intensity Graph & Deep Sleep Intensity Points

Muse S uses EEG to power overnight sleep tracking and deep sleep insights by measuring your delta brain wave activity. This allows you to track the quality and quantity of your deep sleep along with everything from your heart rate to sleep position.

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After each sleep session, you’ll get three personalized graphs that give you different insights into your deep sleep.

The Sleep Stages graph will show you how long you spent in each sleep stage as well as in wakefulness (i.e., when you got up to go the bathroom or were lying in bed reading).

The Deep Sleep Intensity (DSI) graph will show you the total time you spent in a sleep state that meets the EEG criteria to be classified as deep sleep. The DSI graph represents your delta wave activity. This graph shows you the total power of your brain’s delta wave activity between 1 Hz and 4 Hz. Remember that delta wave activity is expected to be greater during deep sleep compared to any other sleep stage which helps sleep researchers classify deep sleep as “deep sleep”. The higher the activity shown on the DSI graph, the greater your sleep intensity is at that moment.

Deep Sleep, Sleep Tracking

Finally, Deep Sleep Points are calculated as the total delta wave activity you accumulated during your sleep. These points are personal to you and are
best used as a reference point when comparing your sleep sessions over time. Monitor your average points or night-by-night point change (ie. increase or decrease) over multiple sleep sessions, and see how shifts in your pre-sleep routine, general sleep hygiene, or changes in the factors listed above affect those points.

Deep Sleep, Sleep Tracking

For example, you may notice that when you have a couple of glasses of wine at dinner, you’re able to fall asleep faster. But studies show that this actually reduces the quality of your sleep by reducing your delta-wave activity and producing a greater number of sleep disruptions later in the night when the effect wears off. As a result, you may see a decrease in Deep Sleep Points, leaving you feeling groggy and unrested in the morning.

Based on your deep sleep insights, you can tweak your sleep routine to fit you best. For example, if you notice you get more deep sleep early in the night, having a good pre-sleep routine could be key to helping you maximize your deep slumber. If your pre-bed routine involves scrolling on your phone or drinking a glass of wine, both could be keeping you from your best Zzz’s.

How to Kickstart Your Personal Deep Sleep Insights and Interpret Your Deep Sleep Score

  1. Do at least 5-7 sleep sessions with overnight tracking and take note of how you feel after each session using the Muse Journal entry feature
  2. After you’ve completed your week of sleep sessions, look back at which mornings you woke up feeling the most refreshed.
  3. For those refreshed sleep sessions, look at the pattern of your DSI graphs. Are there any patterns you see? Is there something consistent or similar happening during these restful nights? This will help give you a sense of what typical restorative sleep looks like for you.
  4. Once you determine what your sleep baseline looks like, you can then observe if you see any deviations from my norm in upcoming sleep sessions.
  5. From here, ask yourself – what might have caused this deviation? Did I go to bed too early or too late? Did I drink caffeine before bed or forget to do my bedtime routine that night? You may find that there is a relationship between what you do during the day or before bed and how refreshed (or not refreshed) you feel when you wake up in the morning.

What Is Deep Sleep?

This stage of sleep also referred to as slow-wave sleep or delta sleep is your deepest slumber where you’re conked out and least likely to wake up. To understand why deep sleep is critical to your health, it’s helpful to review the sleep stages we pass through every night and what happens in each.

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The two main phases of sleep are Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM). Within Non-REM, there are three stages you pass through en route to REM. Every night, we cycle through these four stages several times. REM, hence the name, is your most active sleep stage where your eyes are moving, your heart rate increases and your breathing speeds up. It’s also your dream state. On the other hand, NREM, which includes deep sleep, is the less active phase. Each of its three stages plays an important role in the quality of your snooze (1).

Deep Sleep, Sleep Tracking

Stage 1—Light sleep (N1)

This stage lasts only a few minutes and is the main transition period from being awake to asleep. It’s what happens when you’re reading in bed one minute and in the next, the book is flat on your face. Your heart rate and breathing start to slow down, your muscles relax (and might occasionally twitch), and your brain waves start to slow.

Stage 2—Light sleep (N2)

This stage makes up about 50% of the total sleep cycle and is the stage you may fall into more than any other while you snooze (1). In addition to your breath and heart rate continues to slow, your body temperature drops, your eye movements stop, and your brain waves become consistently slow.

Stage 3—Deep sleep (N3)

In this stage, your heartbeat and breathing become their slowest and your muscles relax. Your brain waves also become the slowest they’ll be while you snooze. Electroencephalography (EEG) results tell us that in this stage your brain is producing more of the slow brain waves known as delta waves.

In general, we spend about 75% of our slumber in NREM sleep and the other 25% in REM. Of this, around 13-23% of our total sleep is deep sleep (2).

Why Do We Need Deep Sleep?

Logging enough hours on the pillow is important, but perhaps even more so is the quality of sleep you’re getting. Deep sleep is critical for short- and long-term memory as well as how you grow and develop. The small, oval-shaped pituitary gland in your brain secretes key hormones, like human growth hormone, to make this happen. Aside from brain health, deep sleep also helps restore your energy, regenerate your cells, boost blood flow to your muscles (necessary for exercise!), promote tissue repair, and strengthen your immune system (1).

Have you ever had that feeling where no matter how much you slept, you still woke up feeling groggy? Or had days when you felt more forgetful and not as mentally sharp? These could all indicate that you didn’t clock enough deep sleep. A lack of deep sleep could also lead to conditions like sleepwalking, night terrors, bed-wetting, and sleep eating (1).

What Are Some Factors That Affect Deep Sleep?

Everything from your age and prescription medications to your hormones and health conditions can affect your sleep. It’s important to consider all these factors when assessing your sleep quality. Whether you’re having trouble dozing off or find yourself tossing and turning at night, here are a few common reasons why that might be (3):

  • You have a sleep disorder. Sleep apnea and insomnia are the most common.
  • You’re stressed or worried.
  • You drank caffeine too late in the day. Caffeine (from coffee, tea, sodas, etc.) blocks a brain chemical called adenosine that helps you sleep.
  • You ate within a few hours of bedtime. Lying down with a full stomach can promote heartburn, making it harder to fall asleep.
  • Your room is not conducive to sleep. Environmental factors like too much light, noisy surroundings, or disruptive partners/pets can all affect your Zzz’s.
  • You were on your devices late. Staring at the blue light from your smartphone can suppress the secretion of melatonin (sleep hormone) and shift your circadian rhythm. Your body interprets the light as daytime and keeps you from getting sleepy.
  • You exercised close to bedtime. Studies show it may be best to finish your exercise at least 90 minutes before you hit the sack. Intense to moderate exercise will also affect you more than lighter exercise, like gentle yoga.
  • You have an inconsistent sleep schedule. Since our body operates best according to our consistent circadian rhythm, when we mess that up, our sleep quality will suffer.

How To Measure Deep Sleep

To measure sleep quality, sleep labs commonly use EEG because of its accuracy. You can also measure your deep sleep with a polysomnography (PSG) test, also known as a sleep test, which includes EEG. For an EEG test, electrodes are placed on the scalp to measure electrical activity in the brain.

Many sleep trackers use heart rate and breathing as a surrogate for estimating sleep. Thus most trackers make a guesstimate as to how much you’re actually sleeping. To get exact data on your sleep habits, an EEG device is necessary. As we mentioned earlier, the EEG can tell you your delta brain wave activity which is the defining aspect of deep sleep (4). EEG essentially allows you to go right to the source and get the raw data.

Deep Sleep, Sleep Tracking


Tips To Get More Deep Sleep

Though there are many factors that influence our sleep including our genes, age, the stress in our lives, and medications, studies widely agree on a few best practices to help you doze off (and stay asleep) (3).

    1. Avoid caffeine after lunch. Caffeine has a 3-5 hour half-life, which means it takes that long for your body to get rid of half the caffeine it consumed. So it could be a good idea to avoid any afternoon pick-me-ups at least 6 hours before bedtime.
    2. Avoid alcohol consumption
    3. Exercise daily and ideally 2-3 hours before bedtime.
    4. Turn off all screens at least 1 hour before bed.
    5. Wake up at the same time every day.
    6. Take a warm shower/ bath or have a cup of herbal tea.
    7. Meditate daily. Science shows meditation can help reduce feelings of stress and anxiety and boost feelings of focus.
    8. Set up your environment for sleep. This includes making sure your room is dark enough (or using a sleep mask), minimizing disruptive noises/pets when possible, keeping your room a comfortably cool temperature, and reserving your sleeping space for sleeping and intimacy only.

Feeling refreshed every morning is not just about the number of hours you sleep, but also about the quality of your rest. Deep sleep is important for everything from processing memories to restoring your energy, so it’s key to make sure you’re getting enough shut-eye at this stage of sleep. By learning what inhibits deep sleep as well as starting to track your sleep overnight, you can discover how you rest best and the daily choices that support your sleep.

Disclaimer: Muse is not a medical device and should not be used to diagnose, treat, or cure any medical conditions. Our products and services are intended to provide information that can help you manage and support your well-being. Please speak with a healthcare provider if you have any concerns about your health or are thinking of making changes to any current prescription medications or lifestyle modifications.



  1. 2018, July 26. What Is Deep Sleep and Why Is It Important? (Blog post). Healthline.
  2. Colten, HR and Altevogt, BM. 2006. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. National Institute of Medicine.
  3. 2020. Unlocking the Power of Sleep. eBook. Muse.
  4. Do Sleep Trackers Really Work? John Hopkins Medicine.
  5. Alcohol’s Effect on Sleep.


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