Sleep Stages

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What is this?

During a sleep session, your body repeatedly cycles through light, deep and REM (rapid eye movement) sleep, as well as short periods awake which you might not even be aware of. Depending on the duration of the sleep session, you will move through up to five sleep cycles, each lasting about 90 minutes on average. Each cycle usually starts with light sleep, followed by deep sleep, then REM sleep, and ends with a brief period awake. However, sleep cycles are not necessarily the same; the early part of your night consists of more deep sleep and the later part more REM sleep. Your transitions between sleep stages and time spent in each of the sleep stages are visualized in a hypnogram.

Why is it important?

Each of the sleep stages plays a different role in your physiological health. Their duration and distribution can reveal important information about your mental and physical well-being. Here is a description of each of the sleep stages:

• Light sleep: During light sleep, you are somewhat alert and can easily be woken.
• Deep sleep: It is more difficult to wake up during this stage. This is when most of your body’s muscle and tissue repair takes place, growth and development are stimulated, immune function is boosted, and energy is restored for the next day. All while you are sleeping like a log.
• REM sleep: This stage is thought to be important for learning and memory, since this is when your brain consolidates and processes information gathered while you were awake so that it can be stored in your long-term memory.
• Awake: See Awake to learn more.
• Unknown: During a sleep session your device might move around on your wrist and lose the heart rate signal, causing some data to go missing. Without this data, it’s impossible to tell whether you were asleep or awake and this is displayed as an unknown state.

Normative ranges

Have a look at this reference table for time spent in each sleep stage per age group and gender in healthy individuals. How do you compare?