Are 90-minute Sleep Cycles Actually 90 Minutes?

90-Minute Sleep Cycles: Are They Actually 90 Minutes?

By Ryan Yost

Chances are you may have heard of the 90-minute sleep cycle. When we sleep we rotate through sleep stages throughout the night, alternating between NREM and REM sleep. First recognized in the 1950s, these are known as ultradian cycles. Not only are they present in brainwaves (EEG) [1-3], but also in heart activity [1,4,5] (HRV specifically [6]) , hormone secretion [7,8], and body movement [1,2,9]. This ultradian cycle was popularized as being 90 minutes long. But with modern advances in biometric data collection, we can measure a bunch of subjects to beg the question: are these ultradian cycles always 90 minutes? How much does it vary?

It turns out that calling an ultradian cycle 90 minutes long is far more inaccurate than calling a soccer match 90 minutes long (avg. of 98 minutes of play time in the 22-23 English Premier League, though the ball was in play for just 52% of that time) [10].

90 minutes is not an accurate reflection of the periodicity of the ultradian cycle. In reality, ultradian cycles typically last anywhere from 95-130 minutes. One study analyzing body movement (using a wrist-worn accelerometer device) showed that across 16,441 nights of sleep from 573 different subjects, ultradian cycle length had a median of 110 minutes, with 75% of ultradian cycles falling between 95 and 130 minutes [11]. Check out the distribution for yourself below:

Another group analyzed 2,312 nights of sleep data from a full polysomnography setup (EEG, EOG, EMG, ECG, pulse ox, nasal airflow, chest movement). They found a mean ultradian cycle length of 116.9 minutes, with a huge standard deviation of 39.7 minutes (skew: 3.5 & kurtosis 22.4) [12]

So what does this mean? Well we shouldn’t really think of sleep cycles as being roughly 90 minutes, but instead as roughly 110 minutes.More accurately 80-150 minutes. This means you can delete your sleep calculator app and not worry about getting sleep in certain increments. There is too much variation in ultradian cycle length for this to matter to your sleep health. This also has implications for daytime focus, as ultradian cycles continue throughout the day as focus vs. relaxation alternate in cycles of roughly… 110 minutes. 

REFERENCES

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