Like so many other viral tendencies in social networks, a seemingly simple formula has gained traction in recent years to improve nighttime rest and ensure you wake up feeling fresh: Calculate sleep in 90-minute blocks. This technique suggests that waking up at the end of these cycles enhances feelings of restfulness and energy for the day ahead. However, scientific evidence raises questions about the reliability of this method.
The Myth of Exact 90-Minute Cycles
A survey conducted in 2024 by the American Academy of Sleep Medicine revealed that nearly one in ten American adults (9%) have tried this technique to determine their sleep and waking times. Online calculators have even been developed to make this calculation easier.
This technique is based on the concept of Natural sleep cycles, which are part of a set of biological rhythms known as Circadian cycles. These cycles dictate sleep, wake, and feeding times, among other bodily functions. According to this method, if someone wants to wake up at 7:00 AM, they should ideally fall asleep at 10:00 PM to complete six full sleep cycles, or at 11:30 PM for five cycles. The rationale is that waking at the end of a full sleep cycle—specifically during REM phase V—helps avoid the grogginess often experienced when waking mid-cycle.
Sleep cycles were first identified in the 1950s when researchers discovered patterns called Ultradian cycles. Throughout the night, the brain alternates between various sleep phases: light sleep, deep sleep, and REM sleep, giving rise to our most intense dreams. Dr. Fouzia Siddiqui, Medical Director of Sentara Rockingham Memorial Sleep Center, noted in an article for Popular Science that while each phase serves specific recovery functions, the oversimplified notion that sleep cycles last precisely 90 minutes does not capture the full complexity of human sleep. The assumption that these cycles uniformly last 90 minutes represents a significant flaw in this theory.
Scientific Reality Dismantles Promises
Recent studies highlight that the length of sleep cycles is not fixed but rather varies significantly among individuals. In fact, an individual may experience longer or shorter cycles throughout one night’s sleep, reflecting changes day by day. A comprehensive analysis of over 16,000 nights of sleep among 573 participants found that the average duration of sleep cycles was not 90 minutes but actually around 110 minutes, with 75% falling between 95 and 130 minutes. Another study based on 2,312 nights of polysomnography determined that the average duration of these cycles was 116.9 minutes.
In essence, designating 90 minutes as a standard for calculating sleep cycles is not only inaccurate but could also be counterproductive, as it increases the risk of waking prematurely. Dr. Andrea Matsumura, an expert in Sleep Medicine and the founder of Sleep Goddess Method, expressed in Popular Science that various factors—such as stress, alcohol consumption, illnesses, or even the time you go to bed—affect sleep cycles. Therefore, adhering to this method might not lead to the expected reduction in sleepiness upon waking.
Additionally, research indicates that as the night progresses, sleep cycles tend to elongate naturally, with earlier cycles lasting between 70 to 100 minutes, while later cycles extend to 90 to 120 minutes. This variability complicates predicting your position in the cycle when it’s time to wake up. In a standard sleep session for a healthy individual, it is improbable to wake from a deep phase of sleep. Phase 3, the deepest and most rejuvenating state, typically lasts between 20 to 40 minutes and occurs mid-session, briefly transitioning back to phase 2, which comprises a lighter state of sleep.
Hence, unless you have severely limited sleep hours, it’s likely you will awaken from a lighter phase, not from a deep one as suggested by the theory of 90-minute blocks, which might leave a lingering feeling of grogginess. Thus, this theory hinges on two genuine facts: waking from phase 3 does result in greater grogginess, and sleep cycles do average around 90 minutes. However, this overgeneralization leads to an oversimplified method cute in its promise but devoid of scientific backing. Instead, the only verifiable approach remains ensuring that individuals receive the total hours of sleep their bodies require, which is still subject to variability.