According to the Division of Sleep Medicine at Harvard Medical School, sleep patterns for adults were once considered an inactive or passive state in which both the body and the brain “turned off” to rest and recuperate from the day’s activities.
Scientists have since found that the brain goes through characteristic patterns of activity throughout each period of sleep, and that it is sometimes more active when we’re asleep than when we’re awake.
Understanding these patterns, and the factors that affect them, can help in making choices that will lead to better quality sleep.
In the early 1920s, scientists regarded sleep as an inactive brain state. It was generally accepted that as night fell and sensory inputs from the environment diminished, so too did brain function. In essence, scientists thought that the brain simply shut down during sleep, only to start again when morning came.
Then in 1929, an invention that enabled scientists to record brain activity from recordings known as EEGs, or electroencephalograms. Researchers could see that sleep was a dynamic behavior, one in which the brain was highly active at times, and not turned off at all.
Over time, sleep studies using EEGs and other instruments that measured eye movements and muscle activity revealed two main types of sleep. These were defined by characteristic electrical patterns in a sleeping person’s brain as well as the presence or absence of eye movements.
Two Main Types of Sleep
The two main types of sleep are rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep.
On an EEG, REM sleep, often called “active sleep,” is identifiable by its characteristic low-amplitude (small), high-frequency (fast) waves and alpha (relaxation) rhythm, as well as the eye movements for which it is named.
Many sleep experts think that these eye movements are in some way related to dreams. Typically, when people are awakened from REM sleep, they report that they had been dreaming, often extremely vivid and sometimes bizarre dreams, or even nightmares as seen in my article, “Sleep And Coronavirus Anxiety – How To Increase Our Quality of Sleep“.
In contrast, people report dreaming far less frequently when awakened from NREM sleep. Interestingly, during REM sleep, muscles in the arms and legs are temporarily paralyzed. This is thought to be a neurological barrier that prevents us from “acting out” our dreams.
Three Sleep Stages of NREM sleep
Prior to 2007, NREM sleep was divided into four stages in the Rechtschaffen and Kales (R&K) standardization of 1968. Then in the 2007 update by The American Academy of Sleep Medicine (AASM), the stages have been reduced to three stages, where stages 3 and 4 were combined into stage 3.
Sleep typically begins with NREM sleep in healthy adults. The pattern of clear rhythmic alpha activity associated with wakefulness gives way to the first stage of sleep, which is defined by a low-voltage, mixed-frequency pattern. The transition from wakefulness to the first stage of sleep occurs seconds to minutes after the start of the slow eye movements seen when a person first begins to nod off. This first period of sleep typically lasts just one to seven minutes.
Second Stage, “N2”. The second stage of sleep, which is signaled by sleep spindles and/or K complexes (both terms defined below) in the EEG recording, comes next and generally lasts 10 to 25 minutes.
Sleep spindles play an essential role in both sensory processing and long-term memory consolidation because they are generated in the TRN (thalamic reticular nucleus) located in the brain. During sleep, these spindles are seen in the brain as a burst of activity immediately following muscle twitching.
Like sleep spindles, K-complexes are defining brainwaves of stage 2 sleep. They differ from sleep spindles in their form. Unlike the rapid burst of activity represented by sleep spindles, K complexes are large waves that react to external stimuli while sleeping. K-complexes also develop later than sleep spindles.
Third Stage, “N3”. As the second stage of sleep progresses, there is a gradual appearance of the high-voltage, slow-wave activity characteristic of the third stage of NREM sleep. This stage generally lasts 20 to 40 minutes and is referred to as “slow-wave,” “delta,” or “deep” sleep. As NREM sleep progresses, the brain becomes less responsive to external stimuli, and it becomes increasingly difficult to awaken an individual from sleep.
Sleep Patterns Through the Night
Following the third stage of NREM sleep, a series of body movements usually signals an “ascent” to lighter NREM sleep stages. Typically, a five to ten-minute period of the second stage of sleep precedes the initial REM sleep episode.
REM sleep comprises about 20 to 25 percent of total sleep in typical healthy adults. For similarities and differences between adults’ and babies’ sleep patterns, see my article, “How Do Babies Sleep? 5 Fascinating Facts to Know“.
NREM sleep and REM sleep continue to alternate through the night in a cyclical fashion. Most slow-wave NREM sleep occurs in the first part of the night; REM sleep episodes, the first of which may last only one to five minutes, generally become longer through the night. During a typical night, the third stage of sleep occupies less time in the second cycle than the first cycle and may disappear altogether from later cycles. Note that sleep cycles do not repeat in stage order.
The average length of the first NREM-REM sleep cycle is between 70 and 100 minutes; the average length of the second and later cycles is about 90 to 120 minutes. The reason for such a specific cycling pattern of NREM and REM sleep across the night is unknown. Some scientists speculate that specific sequences of NREM and REM sleep optimize both physical and mental recuperation as well as some aspects of memory consolidation that occur during sleep, but this has not been confirmed.
Sleep patterns can be affected by many factors, including age, the amount of recent sleep or wakefulness, the time of the day or night relative to an individual’s internal clock, other behaviors prior to sleep such as exercise, stress, environmental conditions such as temperature and light, and various chemicals.
What I’ve learned from doing research for writing this post: Originally, I thought that a good night’s deep sleep meant sleeping 7 or 8 hours straight each night without waking. But after doing research to write this post, I realize that my sleep patterns are normal, and that my sleep stages cycle several times in one night. So why do I generally feel tired when I get up in the morning? Stayed tuned as I explore the topic of sleep further and share my findings with you here on my site. Together, we will make future choices that will lead us to a better quality sleep.
Thank you for reading, and I invite you to leave your comments or questions below, and I will respond soon. Come back again to read more of my informative sleep posts. Bye for now.