Understanding Sleep Cycles and the Stages of Sleep

May 1, 2017

Sleep is a dynamic process. The average healthy adult will experience 3-5 sleep cycles per night. Within those cycles, there are 4 distinct stages of sleep: Wake, Rapid Eye Movement (REM Sleep), Light Sleep, and Deep/Slow Wave Sleep (SWS).

Stages of Sleep

A breakdown of the stages of sleep within each sleep cycle.

Light sleep represents the physiological process taken to transition to deep sleep. Some of the restorative characteristics that define deep sleep occur in this phase, but with less frequency, as your body is more responsive to your environment in light sleep. In fact, there is a theory that light sleep exists to allow the body to be aware of its surroundings and to wake quickly in the event of a threat.

REM sleep is when the brain is restored. It is at this time that ideas and skills acquired during the day are cemented as memories. As it relates to an athlete, any time you are practicing a technical skill, the actual consolidation and retention of that learning happens during REM sleep.

Deep sleep, also known as Slow Wave Sleep (SWS), is believed to be the time when your muscles repair and grow. During this stage the body produces 95% of its daily supply of growth hormones. As an athlete, training sessions break down muscle tissue so that it can rebuild and grow during slow wave sleep.

Wake is included as a sleep stage because it is natural to be awake for brief periods many times in the night. These periods are known as arousals, or “disturbances” in the WHOOP app, and it is normal to experience anywhere from 10-20 per night. These arousals only last a few minutes and you’re not conscious of them, but they matter because you can lose upwards of an hour of sleep in the Wake stage.

Heart Rate in a Typical Night Sleep

Heart rate during, light sleep, REM sleep, SWS, and awake.

Sleep is entered through Light sleep, transitions to SWS within about 10 minutes, then to REM sleep somewhere around 90 minutes after falling asleep. An arousal will follow and a new sleep cycle will begin from there. As mentioned above, a normal night of sleep will contain 3-5 complete cycles, with more possible the longer you sleep.

The amount of time a person will spend in each sleep stage varies night by night. In general, a healthy break down to aim for is the following:

Recommended percentage of time spent in each of the stages of sleep.

Sleep Trends on the WHOOP App

The sleep trend view accessible on app.whoop.com gives an athlete the opportunity to unpack nighttime habits that either help or hurt their sleep efficiency.

When looking at the 2-week average breakdown below, we see plenty of time in bed. However, the ratio of time spent in REM and SWS shows where this athlete is coming up short.

Average time spent in sleep cycles and stages of sleep.

For REM, the athlete is getting 1 hour and 16 minutes on average per night. This is 13% of the total time in bed, glaringly low in comparison to the goal of roughly 22% each night. To assess a root cause, the athlete might look at the night statistics from Feb 3, 4, 5, and 11 (pictured in the bar graph above). For this athlete, a drinking event preceding each low REM sleep seems to be the common thread.

For SWS, this athlete is getting 1 hour and 43 minutes a night. Measured against the average total time in bed, this means that 18% of the athlete’s sleep is spent in the SWS stage. This is a healthy ratio, but if the athlete hopes to investigate behaviors that could contribute to this trend it would be appropriate to select Jan 31, Feb 9, and Feb 10 to see if anything stands out. Here, it appears that the athlete got less than his or her average total amount of sleep on those nights and, on 2 of the 3 occasions, the sleep was following a very high strain day.

Looking for patterns like this can help you build routines that maximize your recovery and improve your heart rate variability.

Tips for Getting Great Sleep

While there is no simple formula for boosting the amount of time you spend in each of the stages of sleep, there are behaviors you can adopt to give yourself the best chance at an efficient night of sleep. Here are some good practices we’ve learned from the leading studies on sleep science:

    • The darker your room, the better you sleep. Light influences the wake stage of sleep so the darker you can get your room, the more time you will get in the restorative stages.
    • Set your room temperature at or around 68 degrees. You will fall asleep quicker when your bedroom is slightly cool.
    • Your bed is not a multi-use space. The more you can train your body to associate your bed with sleep, the more adept you will be at falling asleep in that space. As such, avoid work and leisure time spent in your bed.
    • Screens are stimulating and keep you awake. Avoid using your phone or computer in bed, screen time makes it more difficult to fall asleep.
    • Set a cut-off time for caffeine intake. Caffeine has a lingering presence for many hours after consumption. For greater sleep efficiency, consciously refrain from caffeine consumption at least 4 hours before bed.
  • Plan ahead when consuming alcohol. Just as caffeine intake impacts the body hours after consumption, so too does alcohol. Keep this in mind on a night out so that your sleep performance doesn’t suffer.

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Thea Lee

Thea Lee (19 Articles)

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