Expert Insights from Dr. Anna Pickering a medical writer who received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine.
Our sleep cycles consist of two basic types of sleep: non-rapid eye movement (NREM) sleep, which has three stages, and rapid eye movement sleep (REM). These four stages of sleep comprise one complete sleep cycle, which usually lasts about 90 minutes. Earlier cycles take a bit less time and later cycles take a bit longer than 90 minutes.
“If you’re getting your recommended seven to nine hours of sleep each night, you will cycle through all four stages of sleep five or six times throughout the night,” says Dr. Pickering.
When we complete one cycle, our brains bring us back to the beginning of a new cycle. Then we continue to alternate between NREM sleep and REM sleep as the night progresses. With each successive sleep cycle, we spend more time in REM sleep and less time in NREM sleep, specifically Stage 3 of NREM.
Such a sophisticated sequence of sleep stages surely has its function, but scientists cannot say with certainty what that is. Some researchers theorize that the sequencing and timing of REM and NREM sleep optimize physical and mental restoration as well as some aspects of memory consolidation, but this topic remains controversial.
While there are still questions about sleep, we do know that each stage of the sleep cycle has its own characteristics and purpose, with the three “powering down” stages of NREM being markedly different from the intense brain activity of REM sleep.
Non-REM sleep, which is perhaps best defined negatively as any sleep not recognizable as REM sleep, consists of three separate stages (stage1, stage 2 and stage 3), which are followed in order upwards and downwards as sleep cycles progress.
The three stages of NREM sleep are known as:
Formerly, four stages of non-REM sleep were distinguished, and most older hypnograms therefore usually show four stages of non-REM sleep, rather than three; the distinction can be quite useful at times, and is still quite widely used, even though three stages is now the “official” categorization.
It should be noted that the distinctions between these sleep stages are somewhat arbitrary anyway, and the physiological boundaries between them are necessarily blurred and continuous.
“An electroencephalogram can be used to study the various sleep stages by measuring brain activity during sleep,” explains Dr. Pickering.
Sleep researchers discovered the stages of sleep using an electroencephalogram (EEG) to measure brainwave activity during sleep. The different brainwaves that occur as the night progresses define each sleep stage and are responsible for their distinguishing features.
This stage, sometimes called N1, marks the transition from wakefulness to sleep. In more scientific terms, stage 1 is the period of transition from relatively unsynchronized beta and gamma brain waves (with a frequency of 12-30 Hz and 25-100 Hz respectively), which is the normal range for the awake state, to more synchronized but slower alpha waves with a frequency of 8-13 Hz, and then to theta waves with a frequency of 4-7 Hz.
Brainwaves begin to slow by as much as 50%, along with your heartbeat, breathing, and eye movements. Your muscles begin to relax, and many people experience hypnic jerks or sudden muscle contractions that result in a limb twitch or whole-body startle.
Some people may also experience a falling sensation as they drift off to sleep. Sleep is light during this stage, and you can still be awakened easily. This light sleep stage lasts only 5 to 10 minutes. Typically, this stage represents only about 5% of the total sleep time.
Stage 2 (NREM2 or N2) is the first unequivocal stage of sleep, during which muscle activity decreases still further and conscious awareness of the outside world begins to fade completely. If any sounds are heard, the sleeper is not able to understand their content at this point.
The second sleep stage (N2) is a continuation of the slowing process begun in N1. Your heartbeat and breathing slow further, slow eye movements come to a complete halt, and your body temperature drops as it prepares to enter the deep sleep of Stage 3. The most notable features of N2 sleep are brief bursts of intense and rapid brain activity called sleep spindles.
Researchers have shown that sleep spindles clear a path for new learning. In their study, Matthew Walker and his colleagues at the University of Berkeley demonstrated that sleep spindles move fact-based memories from areas of limited storage to areas with greater capacity for information and long-term memories.
Because sleepers pass through this stage several times during the night, more time is spent in stage 2 sleep than in any other single stage, and it typically constitutes about 45%-50% of total sleep time for adults (or even more in young adults).
Stage 2 sleep accounts for about 10 to 25 minutes per sleep cycle and occurs for longer durations in the second half of the night. That means if you cut your total sleep time short by even an hour or two, you may not be able to learn as much as you could on a solid seven to nine hours of sleep.
K-complexes also occur exclusively during N2 sleep. K-complexes are large brainwaves that are drastically different from the brain activity before and after them. They often occur in response to sounds or other disturbances in our environment while we sleep. Sleep experts believe that the combination of K-complexes and sleep spindles help you stay asleep and allow you to enter the deep sleep that follows N2.
Stage 3 (NREM3 or N3) is also known as deep or delta or slow-wave sleep (SWS), and during this period the sleeper is even less responsive to the outside environment, essentially cut off from the world and unaware of any sounds or other stimuli. This stage marks the deepest sleep of the night. Your heartbeat and breathing slow to their lowest levels during this stage. Brain activity is also slow, and you become much harder to arouse in N3.
“In stage 3 of the non-REM sleep cycle, parasomnias like sleep walking, sleep talking, and night terrors can occur as this is the deepest sleep stage,” adds Dr. Pickering.
This stage usually lasts 20 to 40 minutes and decreases in duration in successive sleep cycles. During N3, your body repairs and regrows tissue, builds muscle and bone, and strengthens the immune system. As a result, sufficient N3 sleep is necessary for you to feel refreshed in the morning.
People under 30 years old get in total about two hours of this deep sleep each night, but your time spent in N3 sleep decreases as you get older. After the age of 65, you may spend as little as 30 minutes per night in deep sleep.
The fuzziest and most arbitrary distinction historically has been between stages 3 and 4, with stage 4 characterized only by somewhat higher delta brainwave levels. The former N4 is now considered a deeper level of N3 sleep.
After we complete all three stages of NREM sleep, we briefly enter the lighter sleep of N2 before entering a period of rapid eye movement (REM) sleep. The most well-known characteristics of REM sleep are the quick eye movements under closed eyelids for which it is named and the presence of vivid and often memorable dreams.
Related: What is REM rebound?
For years sleep experts agreed that dreams occurred exclusively during REM sleep. Recent studies, however, have found that dreams do indeed occur during NREM sleep, although they appear to be slightly less frequent, tend to be less intense, and we usually don’t remember them.
These findings mark a revolution in our thinking about dreams, but many questions remain about why and how we dream as well as the many functions of our stages of sleep.
Dr. Anna Pickering is a freelance medical writer based in Portland, OR working for The Med Writers. She received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine. Before transitioning to writing she completed a year of postdoctoral research at Oregon Health and Science University and worked briefly at the biotech startup Ayumetrix.
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