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Understanding Sleep Cycles and the Stages of Sleep

November 1, 2019

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

Stages of Sleep: Light, Slow Wave Sleep (SWS), REM, Wake

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.

Slow Wave Sleep (SWS), also known as deep sleep, is 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.

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.

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. While they only last a few minutes and you’re not conscious of them, you can lose upwards of an hour of sleep in the Wake stage due to disturbances.

Heart Rate in a Typical Night’s Sleep Cycle Stages

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

Time Spent in Each Stage of 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:

How much of your sleep should be deep/slow wave (SWS), REM and light.


Learn More: How Much Time Should You Spend in Each Stage of Sleep?

Analyzing Sleep Stages & Trends with WHOOP: Slow Wave Sleep vs REM

The long-term sleep trends accessible on the WHOOP web app (app.whoop.com) give you the opportunity to unpack nighttime habits that either help or hurt your sleep efficiency. Here’s an example of one WHOOP athlete’s data:Trend view of time spent each night in sws sleep (deep), rem, light and wake.

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:Comparing average time spent in rem vs sws 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 (here’s a deep dive into how alcohol affects your sleep).

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 their 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 WHOOP recovery and improve your heart rate variability, one of the key metrics for determining your body’s readiness to perform each day.

Tips for Getting Great Sleep & Improving Sleep Cycles

While there is no simple formula for boosting the amount of time you spend in any 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.

Fall asleep and wake up at similar times each day. We call this sleep consistency, and studies have shown it can improve the quality and efficiency of your time in bed (check out The Circadian Rhythm Sleep Hack).

Listen to the WHOOP App Sleep Coach. Based on your physiological data and natural circadian rhythm, the Sleep Coach makes daily recommendations for your optimal time to go to bed at night and get up the next morning.

More: Sleep Tips from the 100 Best Sleepers on WHOOP

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