Gina Johnson
The Apple Watch's Sleep app can help you understand your sleep patterns by tracking your sleep stages. The app categorises your sleep into three types: REM, core, and deep sleep. While REM and deep sleep are well-known, the term core sleep is unique to Apple and refers to light sleep or the first two stages of the sleep cycle, N1 and N2. This is in contrast to the scientific understanding of sleep stages, where core sleep is not a recognised stage and can refer to a portion of the night that includes both deep and light sleep. Despite this discrepancy, the Apple Watch's sleep tracking feature can provide valuable insights into your sleep habits and help you set sleep goals. However, it's important to note that wearables like the Apple Watch may not always accurately distinguish between different sleep stages.
| Characteristics | Values |
|---|---|
| Core Sleep | Light sleep or non-REM sleep stages 1 and 2 |
| Deep Sleep | Non-REM sleep stage 3 or slow-wave sleep |
| REM Sleep | Rapid Eye Movement sleep |
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What You'll Learn
Core sleep is light sleep
The Apple Watch's Sleep app can estimate the time spent in different sleep stages, categorising sleep into core, deep, and REM. While REM and deep sleep are well-known, the term "core sleep" is more confusing and is used differently by Apple.
Apple's definition of "core sleep" is identical to the scientific definition of "light sleep". It is also known as N2 sleep. However, Apple chose to use the term "core" instead of "light" to avoid possible misunderstandings, as light sleep might be assumed to be less important than deep sleep.
Core sleep, or light sleep, makes up the first two stages of each sleep cycle and constitutes the majority of sleep time. However, it is the least restorative type of sleep. During this stage, the body begins to relax, and brain waves start to differ from those observed during wakefulness. While core sleep is essential, it is better to focus on getting sufficient deep or REM sleep, which are more crucial for restoration and cognitive functions.
The Apple Watch uses an accelerometer, heart rate sensor, and blood oxygen sensor to approximate sleep stages. However, it may struggle to distinguish between N1 and N2, leading to some core sleep being misidentified as deep sleep or vice versa. Therefore, while the Apple Watch can provide insights into sleep patterns, it is important not to rely solely on its categorisations when assessing sleep quality.
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Deep sleep repairs the body
Sleep is a natural process that allows the body to rest and repair itself. During sleep, the body powers down, and most body systems, including the brain, become less active. This reduced activity makes it easier for the body to carry out essential repairs and maintenance.
Deep sleep, also known as slow-wave sleep, is the third stage of the sleep cycle. It is critical to restorative sleep and typically occurs during the first half of the night. During this stage, the body releases growth hormones, repairs muscles and tissues, strengthens bones and joints, and reinforces the immune system. The intensive repair work that occurs during deep sleep is why it is harder to wake someone up during this stage.
While the Apple Watch categorises sleep into core, deep, and REM sleep, it is important to note that these categories are based on the well-known sleep cycle. Core sleep, as defined by the Apple Watch, refers to light sleep or stages N1 and N2, which are the first two stages of the sleep cycle. Deep sleep, or stage N3, follows, and is when the body engages in repair and restoration.
The amount of deep sleep an individual experiences can vary due to factors such as age, health, medications, and environment. On average, the Apple Watch user gets 49 minutes of deep sleep per night, which is about 13% of their total sleep time. However, this can range from 7% to 18% of total sleep time, with the latter placing individuals in the top 10% of deep sleepers.
Getting sufficient deep sleep is crucial for overall health and well-being. Lack of sleep or accumulated sleep debt can have negative consequences, such as an increased risk of developing Type 2 diabetes and other health issues. Therefore, it is important to prioritise sleep and aim for a healthy amount of deep sleep to allow the body to repair and restore itself optimally.
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REM sleep repairs the mind
Sleep is divided into cycles that last about 90 minutes, and each cycle has four stages: N1, N2, N3, and REM. The Apple Watch divides your sleep into three categories: core, deep, and REM. Core sleep is the equivalent of light sleep and takes place in the first few hours of sleep. It is the most essential sleep stage, but it is the least restorative. N1 and N2, which are sometimes called "light sleep", are being labelled as "core sleep" by the Apple Watch. N3 is "deep sleep", which is the third stage of sleep and is critical to restorative sleep.
REM sleep is believed to repair the mind and is important for memory, learning, and creativity. It stands for rapid eye movement and occurs at intervals during the late part of the night. It is considered optional sleep that is less crucial. The average Apple Watch user spends 20% of their time in REM sleep, while four out of five people spend between 15 and 25% of their time in REM.
The Sleep app on the Apple Watch allows you to create sleep schedules to help you meet your sleep goals. The watch can estimate the time spent in each sleep stage—REM, core, and deep—as well as when you wake up. When you wake up, you can open the Sleep app to learn about your sleep trends over the past 14 days. Sleep tracking is available on watchOS 8 or later, and you can turn it on in the Settings app.
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Sleep tracking on Apple Watch
Sleep tracking is a feature on Apple Watch that allows you to track your sleep patterns and set sleep goals. The Sleep app on the Apple Watch divides sleep into three categories: Core sleep, Deep sleep, and REM sleep. These categories are based on the well-known sleep cycle, which consists of four stages. Core sleep, or light sleep, is the first stage of sleep and is the most essential stage. It is also the majority of sleep time, averaging 67%. Deep sleep, or slow-wave sleep, is the second stage and is critical for restorative sleep. This is when the body repairs muscles and the immune system. REM sleep, or rapid eye movement sleep, is the final stage and typically when dreaming occurs. It is believed to be important for memory, learning, and creativity.
Apple Watch uses an accelerometer, heart rate sensor, and blood oxygen sensor to approximate sleep stages. However, it is not always accurate in distinguishing between N1 and N2 sleep, which are both categorized as Core sleep. While it can be useful for understanding your sleep patterns, it is important to note that wearables like the Apple Watch are not perfect at determining the exact stage of sleep you are in. They are generally good at differentiating between being asleep and being awake.
To use the sleep tracking feature on the Apple Watch, you need to wear your watch to bed and ensure it has sufficient battery life. The Sleep app will then estimate the time spent in each sleep stage and provide sleep trends over the past 14 days. You can also set up sleep schedules and create multiple schedules, such as one for weekdays and another for weekends. Sleep Focus is a feature that turns off the watch display and limits distractions before your scheduled bedtime. Charging reminders can also be set up to ensure your Apple Watch has enough battery life to track your sleep.
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Sleep stages and cycles
Sleep is divided into cycles that last about 90 minutes, and each cycle has four stages. The Apple Watch divides your sleep into three categories: core, deep, and REM sleep. However, the scientific understanding of sleep stages, based on brain wave patterns that can be read with an EEG (electroencephalogram), includes more specific stages.
Core sleep, according to the Apple Watch, is another name for light sleep, which scientists refer to as stages N1 and N2. These stages are often combined by the Apple Watch due to the difficulty in distinguishing between them. Core sleep is the majority of sleep time, but it is the least restorative. It is the first few hours of sleep, during which your body begins to relax and your brain waves start to change from how they appear when you are awake.
Deep sleep, or slow-wave sleep, is the third stage of the sleep cycle. It is harder to wake someone up during this stage. This stage is critical for restorative sleep, as it is when the body repairs muscles and the immune system. It is also when the body releases growth hormones, which help to build and repair bones and tissues. The Apple Watch approximates that deep sleep makes up about 12-13% of total sleep time, but this may be an underestimation.
REM sleep, or rapid eye movement sleep, occurs at intervals during the latter part of the night. It is typically when you dream and is believed to be important for memory, learning, and creativity. While it is considered less crucial than deep sleep, it is believed to repair the mind. The average Apple Watch user spends about 20% of their time in REM sleep, which is within the range of 15-25% for most people.
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Frequently asked questions
REM stands for rapid eye movement and occurs during the late part of the night. It is believed to be important for memory, learning, and creativity.
Core sleep is the equivalent of light sleep and takes place in the first few hours of sleep. It is the most essential sleep stage but the least restorative.
Deep sleep is the phase of the sleep cycle where it is harder to wake someone up. It is when the body releases growth hormones and works to build and repair muscles, bones, and tissue.
The Apple Watch uses its accelerometer, heart rate sensor, and blood oxygen sensor to track sleep. It can estimate the time spent in each sleep stage—REM, core, and deep—as well as when you wake up.
The Apple Watch is pretty good at telling the difference between being asleep and being awake. However, it can easily miscategorize some of your light sleep as deep sleep and vice versa.
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