7.3 The Basics of Sleep: Stages and Rhythms

Our bodies cycle between two types of sleep: non-rapid eye movement sleep and rapid eye movement sleep. A sleep session starts with non-rapid eye movement or NREM sleep. Four distinct stages make up NREM sleep and comprise 75-80% of total sleep time. The other 20-25% of sleep time is spent in rapid eye movement or REM sleep, which is considered the fifth stage in a sleep cycle. The first NREM-REM cycle lasts between 70 and 100 minutes and each subsequent cycle lasts between 90 and 120 minutes (Carskadon & Dement, 2005). How many cycles a person experiences depends on their total hours of sleep.

NREM Sleep

Stage 1 NREM sleep is that hazy period when we are just falling asleep. This light sleep that can be easily disrupted. The eyes roll slowly back and forth and the body’s muscles become relaxed. Stage 1 lasts between two to seven minutes for most people and is the period when a sleeping person is most susceptible to disturbances, like noises. Have you ever woken yourself up by jerking in your sleep? Maybe you felt like you were falling? This is known as a hypnic jerk and is a result of sudden muscle contractions during stage 1 NREM.

Stage 2 NREM sleep typically lasts for about 10 to 25 minutes and gets longer during later cycles in a sleep session. It comprises about half of an adult’s total sleep time. Heart rate and body temperature decrease during stage 2, and while a sleeper in this stage can still be roused quite easily, it requires slightly more effort compared to stage 1. During stage 2, studies show bursts of brain activity called sleep spindles, named after how they appear on an EEG reading. Sleep spindles are thought to be important for memory consolidation (Warby et al., 2014). In one study, research participants who learned a new task had sleep spindles of significantly higher density compared to participants who did not learn a new task (Gais et al., 2002). Also apparent in EEG readings of stage 2 NREM are higher amplitude waveforms known as K-complexes. Though their purpose and cause are still not fully understood, researchers theorize that K-complexes serve sleep-protective purposes by deepening sleep (Forget et al., 2011).

Stage 3 NREM and Stage 4 NREM are very similar and are sometimes collectively referred to as slow-wave sleep or deep sleep. Stage 3 NREM sleep typically lasts about five minutes and stage 4 generally lasts about 20 to 40 minutes. Stage 4 is longest during earlier cycles in a sleep session. Stage 4 NREM sleep is the deepest sleep and when the body does most of its repair work. A person in this stage is not likely to be easily roused. The body craves this deep sleep when sleep deprived and will try to compensate for lost sleep by moving faster into and staying in stage 4 longer. As a result, the body can miss out on the memory consolidation that takes place in stage 2.

The 5th stage of sleep, REM, is named for the fluttering and back and forth movements of the eyes. REM sleep typically lasts about five minutes, but lengthens during later cycles. It is usually easier to wake here than in stage 3 or 4 NREM sleep. In fact, brief awakenings often occur around the transitions to and from REM sleep.

REM sleep is very important in learning and memory processing (Cai et al., 2009). Even though your body is still, the brain is highly active during REM sleep, similar to when you’re awake, which is probably why the most vivid and pronounced dreaming occurs during this stage. REM sleep is also characterized by increased heart rate and blood flow to the brain, a rise in blood pressure and metabolism, a faster respiration rate, and increased oxygen consumption. People even become sexually aroused during REM sleep.

The following images show graphically what the cycles and composition of a typical sleep session might look like. The example hypnogram below illustrates the typical stages of sleep of a healthy adult. Over the duration of the sleep session, most of the time is spent in stage 2 NREM sleep, and the majority of the remainder is split between REM and stage 3 and 4 NREM sleep, with only a small amount of time spent in Stage 1 NREM sleep. The accompanying pie chart shows a sample breakdown of the percent of time spent in each stage.

Sleep-Wake Regulation and the Circadian Rhythm

It is thought that sleep is regulated by two main processes: sleep-promoting processes and processes for maintaining wakefulness. Our need for sleep builds throughout our waking hours and peaks before we go to bed; this need for sleep decreases as we sleep. The maintaining wakefulness process is regulated by the circadian system and works directly against the sleep-promoting process.

Circadian rhythm refers to the cycle that lasts about a day and functions as an internal or biological clock. Our circadian rhythms are governed by the suprachiasmatic nucleus—or SCN. The SCN picks up cues of lightness and darkness from the retina at the back of the eyes (Wright et al., 2012), which directs the body’s schedule for sleeping and eating, as well as producing certain hormones. Melatonin is the primary hormone that causes sleepiness, though it doesn’t impact actual sleep. Our bodies naturally get sleepy when it’s dark and feel more alert when it’s light, thus why most people typically feel most sleepy at night. Changing our internal clocks or adjusting to new cues for day and night is difficult and can take several days or weeks.

Delayed sleep phase syndrome is a chronic delay in a person’s circadian rhythm and is characterized by falling asleep much later—sometimes 2 or 3 am—and waking later in the day compared to typical sleep-wake times. The exact cause or prevalence of delayed sleep phase syndrome is unclear, though some research indicates it may be more common for adolescents and young adults (AASM, 2005), hence the reason this syndrome is part of the movement behind later school start times.

Learning Activity: Types and Stages of Sleep

Over the course of the night someone who’s sleeping goes through a number of stages comprised of two types of sleep; NREM (non-rapid eye movement) and REM (rapid eye movement).

Click on each sleep stage below to learn more about it.