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What Are The Stages Of Sleep & What Does Each Stage Do?
Clock against a dark background with moon and stars, suggesting peaceful slumber and serene dreams.
During wakefulness, certainneurons fire in our brains, making us aroused and alert. However, during sleep, these neural circuits are inhibited, our muscles are completely relaxed, and our body becomes completely inactive (Schwartz & Roth, 2008).
Introduction to Sleep
A sleeping person, for example, is in a state of unconsciousness; so is an individual in a coma or someone who has fainted. The difference, however, is that a sleeping person can be aroused if the stimulus is strong enough (for example, shaking the person, shining a bright light, or making a loud noise).
Once awake, you are back in a state of consciousness, aware of your surroundings.
The Circadian Rhythm
But how do our bodies know when to go back and forth between sleep and wakefulness? Luckily, we all have an internal clock telling us when to do so. The circadian rhythm, our 24-hour clock, operates as our sleep/wake cycle.
This body clock is located in the suprachiasmatic nucleus (SCN), a pair of distinct groups of cells located in the hypothalamus (Dubokovich, 2007).
When there is less light (like at night), the SCN tells the brain to make more melatonin so you get drowsy and are able to fall asleep. After a night’s rest, melatonin levels will drop, and you will be awake for the day.
REM and Non-REM Sleep
Sleep has been traditionally divided into two categories: Non-rapid eye movement (NREM) and rapid eye movement (REM).
Non-REM Sleep
During non-REM sleep, electrical activity in the brain slows, growth hormone secretion occurs, and there is a decrease in muscle activity, heart rate, respiration, and oxygen consumption (Purves et al., 2001).
REM Sleep
REM sleep occurs after the brain passes through stages one, two, and three and typically occurs approximately every 90 minutes (McCarley et al., 1995).
During REM sleep, brain activity increases, voluntary muscles are inhibited, and rapid eye movements and dreams occur (McCarley et al., 1995). The next section will go into more detail on REM sleep as well as the characteristics of the specific stages of non-REM sleep.
Sleep StagesIn 1957, William Dement, along with Nathaniel Kleitman, came out with another revolutionary paper that discussed the specific sleep stages that together form the internal cycle that occurs every night we sleep (Dement & Kleitman, 1957).
Sleep Stages
In 1957, William Dement, along with Nathaniel Kleitman, came out with another revolutionary paper that discussed the specific sleep stages that together form the internal cycle that occurs every night we sleep (Dement & Kleitman, 1957).
Stage 1: lightest sleep
Your brain waves also begin to slow down as you have entered a stage of light sleep.
In the beginning of stage 1, the brain produces high-amplitude alpha waves and begins to produce theta waves as the stage progresses (Abeln et al., 2014). Put simply; brainwaves are electrical pulses in the brain that change according to what we are doing or how we are feeling (Abhang et al., 2016).
As the waves slow down (a decrease in frequency or cycles per second), the brain goes into a deeper sleep.
Alpha waves are the highest frequency (thus, the fastest) of the three brain waves that characterize sleep, explaining why, when we have just fallen asleep and are not yet in a state of deep sleep, we can be easily awoken.
Stage 2: light sleep
In this stage, your body temperature drops, and your eye movements stop completely (Lockett, 2020).
In addition to sleep spindles is the presence of k-complexes, high amplitude patterns of brain activity that may occur in response to environmental stimuli (Forget et al., 2011). In total, N2 lasts roughly 20 minutes long (Altevogt & Colten, 2006).
Stages 3 & 4: deep sleep
Formerly known as stages 3 and 4, stage 3 (N3) is the final stage of non-REM sleep. During N3, your heartbeat and breathing slow to their lowest levels, and your muscles are so relaxed that it may be hard to awaken you (Lockett, 2020).
This stage is referred to as “slow wave sleep” due to the presence of delta waves, or slow brain waves, as the brain is now in a deeper state of sleep as compared to N1 and N2 (Abeln et al., 2014).
Sleepwalking and night terrors are also unique characteristics of N3 (Singh et al., 2018).
Sleepwalking, or somnambulism, occurs when sleeping and waking are combined, and the individual wanders around in a dazed and uncoordinated state (Kales et al., 1980). Night terrors are partial waking from sleep during which behaviors such as screaming, kicking, and panic occur (Kales et al., 1980).
Both sleepwalking and night terrors more commonly happen in children than adults. Slow-wave sleep is also associated with bodily recovery and certain types of learning (Tononi & Cirelli, 2006).
Additionally, it is in this stage that human growth hormone (HGH) is released. HGH works to restore and rebuild your body and muscles from the stresses of the day (Iyo, 2020). N3 lasts between 20 to 40 minutes long (Altevogt & Colten, 2006).
Stage 5: REM sleep
REM sleep is a much deeper sleep than any of the three stages of non-REM sleep.
EEG is an electrophysiological monitoring methodto record the electrical activity of the brain, and this technology is commonly used in sleep studies to measure brainwave activity (Niedermeyer & da Silva, 2004).
However, REM rebound, or the increase of REM sleep above normal levels, can occur after a period of sleep deprivation (Verma et al., 2001). That is, there is an increase in REM sleep after a night of little REM sleep.
A key characteristic of REM sleep is the presence of dreams. Dreams are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind (Tirapu-Ustarroz, 2012).
While the meaning of dreams and why we dream is still relatively unknown, theories suggest that we may dream as we consolidate memories (Pace-Schott, 2013), prepare for possible future threats (Valli et al. 2005), reflect unconscious mental function in a psychoanalytic way (Eiser, 2005), or help further develop cognitive capabilities (Medrano-Martinez & Ramos-Platón, 2014).
The Sleep Cycle
Your body cycles through these stages four to five times each night. Cycles earlier in the night tend to have more NREM sleep, while later cycles have a higher proportion of REM.By the final cycle, your body may even skip NREM deep sleep entirely.
Your body cycles through these stages four to five times each night. Cycles earlier in the night tend to have more NREM sleep, while later cycles have a higher proportion of REM.
By the final cycle, your body may even skip NREM deep sleep entirely.
Overall, your body spends more time in the NREM phases of sleep
Sleep Cycles in Other Animals
Humans may be unique in a lot of ways, but the fact that we sleep is not one of them. As discussed, sleep is important for recovery, memory storage, and growth, so it makes perfect sense that other animals need sleep, too.
However, the length of sleep, brain’s state of consciousness, and whether dreaming occurs differs among species.
But do all animals even sleep? Research has shown that birds and mammals sleep (Siegel, 2008).
That is, they become unconscious of their surroundings for a certain period of time. Reptiles sleep, but research is inconclusive regarding whether they reach a REM sleep-like state.
Fish and amphibians reduce their state of awareness but do not ever become unconscious (Siegel, 2008). Insects, on the other hand, do not appear to sleep (and have never been shown to enter REM sleep), although they may experience periods of inactivity (McCarley et al., 1995).
It is important to understand that while other animals also sleep, different types of animals have different sleep cycles.
Birds and mammals share non-REM and REM sleep, but for birds, both cycles are much shorter – non-REM averages roughly two and a half minutes, while REM lasts about only nine seconds (Ogden, 2015).
The length of these cycles also ranges from mammal to mammal. For example, REM sleep occurs for 24 minutes in a cat and 12 minutes in a rat (McCarley et al., 1995).
Although research on reptile sleep cycles is not completely conclusive, a 2016 study on the brain of a lizard, the Australian dragon, revealed that slow wave and REM sleep patterns oscillated continuously with a period of roughly 80 seconds each (Shein-Idelson et al., 2016).
Now that we know other animals also sleep, and some even alternate between cycles of REM and non-REM, just as humans do, you might be wondering if other animals dream.
By studying brainwaves and stages of sleep, research supports the claim that both mammals and birds dream, as both groups enter REM sleep where dreaming occurs.
It is possible that reptiles dream since past studies reveal they also exhibit some form of REM sleep (Libourel et al., 2018; Shein-Idelson et al., 2016), but researchers are still uncertain as to whether this is completely true.
And while it is difficult to discern whether animals do dream, it is even more challenging to understand the specific content of these dreams. It becomes clear that further research is necessary for this field.
When Should I Set My Alarm?
An individual sleep cycle typically lasts around ninety minutes to two hours, during which the brain cycles from slow-wave sleep to REM sleep. However, the sleep cycle is not as simple as cycling through the stages sequentially.
As the night progresses, the amount of time we spend in specific stages changes, as does the order of the stages.
For example, the average length of the first non-REM-REM sleep cycle is 70 to 100 minutes.
The second and subsequent cycles last longer – approximately 90 to 120 minutes (Carskadon & Rechtschaffen, 2011).
In adults, REM sleep increases as the night progress and is the longest in the last one-third of the time spent asleep. N2 begins to account for the majority of non-REM sleep, and N3 can even disappear altogether (Altevogt & Colten, 2006).
On an average night, adults typically need to complete at least four or five sleep cycles per night, or 7 to 9 total hours of sleep (Hirshkowitz et al., 2015). However, there is a large degree of variability from person to person and from night to night (Carskadon & Rechtschaffen, 2011).
Babies, for example, have shorter sleep cycles, lasting only about 50 minutes for the first nine months of life, and it is typical for newborns to sleep anywhere from 14 to 18 hours a day (Hirshkowitz et al., 2015).
Most professionals agree that shorter naps are better if a person’s goal is to wake up feeling refreshed and alert. However, the results of a 2019 study indicated that 25-, 35-, and even 45-minute naps significantly reduced signs of stress and fatigue in physically active men (Hsouna, 2019).
It also improved their attention and physical performance. The National Sleep Foundation warns that taking longer naps may leave you feeling groggy, as you will need to wake up from a deeper sleep.
In sum, if you want to take a nap, make sure it isn’t too short that you don’t wake up feeling refreshed or too long that you interrupt your REM sleep and feel even more tired than you did before your nap. Just like in Goldilocks, the length of your nap must be just right!
And when it is time to go to sleep for the night, make sure you are giving yourself a minimum of 7 hours of sleep. The bottom line is that sleep is important, and you should make sure to get enough sleep every day!
William Dement: The Father of Sleep
How do we know anything about sleep if we are, well, asleep? We can’t administer a questionnaire mid-nap or have people have a conversation while unconscious. Fortunately, we have a man named Dr. William Dement to thank for most of what we know about sleep.
Dr. Dement, known as the “Father of Sleep Medicine,” is a Washington state native who had dreams of becoming a journalist.
Unfortunately, all of the journalism classes at the University of Washington were full, so Dement opted to enroll in an Introduction to Psychology course instead.
He found this class to be so interesting that he scrapped his plans of becoming a journalist and decided he wanted to become a psychoanalyst (Stanford, 2008).
After attending the University of Washington, Dement went on to the University of Chicago School of Medicine, where the only person studying sleep was faculty member Nathaniel Kleitman (Stanford, 2008).
The two began to work together in the 1950s, making some of thegreatest sleep discoveries in the field. Their first discovery, in 1953, was that of rapid eye movement (REM) sleep (Aserinsky & Kleitman, 1953).
FAQs
How long is a sleep cycle?
How long is a REM cycle?
How do brain waves change as a sleeper progresses from stage 1 sleep to rem sleep?
In contrast, during REM sleep, brain waves become faster and more similar to the waves observed during wakefulness. This is when vivid dreaming occurs, and the brain shows increased activity and resembles the alert state. These transitions between sleep stages reflect different aspects of sleep and the corresponding brain activity.
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Saul McLeod, PhD
BSc (Hons) Psychology, MRes, PhD, University of Manchester
Saul McLeod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.
Olivia Guy-Evans, MSc
BSc (Hons) Psychology, MSc Psychology of Education
Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.
Charlotte RuhlResearch Assistant & Psychology GraduateBA (Hons) Psychology, Harvard UniversityCharlotte Ruhl, a psychology graduate from Harvard College, boasts over six years of research experience in clinical and social psychology. During her tenure at Harvard, she contributed to the Decision Science Lab, administering numerous studies in behavioral economics and social psychology.
Charlotte RuhlResearch Assistant & Psychology GraduateBA (Hons) Psychology, Harvard University
Charlotte Ruhl
Research Assistant & Psychology Graduate
BA (Hons) Psychology, Harvard University
Charlotte Ruhl, a psychology graduate from Harvard College, boasts over six years of research experience in clinical and social psychology. During her tenure at Harvard, she contributed to the Decision Science Lab, administering numerous studies in behavioral economics and social psychology.
