The Sleep Cycle, Explained
We experience approximately four to six sleep cycles a night, says Dr. Allison Siebern, PhD, CBSM, Proper's head sleep science advisor. Every 70 to 90 minutes or so, we phase through a cycle that includes both non-REM and REM sleep. "We do not stay in one particular stage of sleep very long," she explains. "All of the stages are important for various reasons, so even though we call N3—which stands for NREM Stage 3 or deep sleep—it does not mean it is the only important one." Ahead, we dive deeper into the many phases of the sleep cycle; use this knowledge to improve the quality of your rest.
There are several parts of the sleep cycle that are referred to as quiet sleep, or NREM or non-REM sleep (non-rapid eye movement). They consist of the N1 through the N3 phases. "N1 sleep is like a portal: It lasts just a few minutes when we initially drop into sleep, before we transition into the next stage," Dr. Siebern says. "This stage accounts for only two to five percent of our night's sleep." In this phase, the body begins to relax, and your heart and respiratory rate slow. "Then there is N2 sleep, which comprises 45 to 55 percent of our total night's sleep," she explains. "During this time, our heart rate and respiratory rate slow even further."
During the N3 phase, you experience what is called "delta" or "slow-wave sleep" (SWS). "The brain generates more low-frequency, high-amplitude delta waves during this time. Heart rate and respiration continue to decrease," shares Dr. Siebern. "N3 accounts for approximately 20 percent of the night's sleep—and is front loaded in the early sleep cycles and tends to disappear the second half of the night." While your body lingers in N3, your immune system repairs and restores itself, which makes it an important part of your nightly snooze.
The REM (or rapid eye movement) stage follows N3, according to Dr. Siebern, and occurs as the final stage in the cycle. "REM has a brief period in the initial sleep cycles and has a more prolonged duration during the second half of the night," she says. "REM accounts for about 20 to 25 percent of our time asleep." While our brain waves tend to be more active during REM sleep, our muscles remain fully relaxed. "This is when dreams typically occur, and our relaxed physical state keeps us from acting those out," she says, adding that this phase benefits cognitive functions such memory consolidation, creativity, and learning.
Dr. Siebern explains that the sleep cycle typically progresses from N1 through N3 to REM. "This cycle repeats four to five times throughout the night. For example, N1, N2, N3, and REM, multiplied by four or five," she says. "However, N3 can disappear from later cycles." In addition to a vanishing N3 phase, some people may experience brief, normal awakenings; they are so quick, that we rarely remember them.