There are two types of sleep, Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM), and they occur in four stages, with NREM having 3 stages and REM being its own stage. While sleeping, your brain usually goes through cycles of these four stages about 4-6 times a night!
Each cycle lasts approximately 70-120 minutes, with an average of 90 minutes. Each repetition of the sleep cycle lasts longer and takes you into a deeper sleep. Normally, you will spend 75% of your total sleep time in NREM and the remaining 25% in REM sleep.
This hypnogram shows how these sleep cycles look: As you can see, your brain moves through As you sleep, your brain journeys through these four stages.
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The sleep stages generally involve changes in muscle tone, brain activity, and eye movements .
How Many Sleep Stages Are There
Each stage of sleep plays a unique role in maintaining brain health and performance by making needed repairs and maintenance.
- Stages 1 through 3 are a type of sleep known as non-rapid eye movement (NREM) sleep. These stages are also described as quiet sleep.
- Stage 4 is a type of sleep known as rapid eye movement (REM) sleep. It's also known as active sleep.
Sleep Fact: Wait. Aren't there five stages of sleep?
The American Academy of Sleep Medicine used to recognize five stages of sleep, but sleep was redefined to four stages in 2007.
Taking a Closer Look at Sleep
A lot of what scientists know about the nature of sleep comes from studies of volunteer patients who spent the night in a sleep lab hooked up to an electroencephalograph, more commonly called an EEG. The neurons in your brain "talk" to each other through waves of energy. EEG records five different speeds of brain waves.
From the fastest waves in light sleep to the slowest waves in deep sleep, these types of brain waves are called:
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When you are awake, your brain produces beta and gamma waves, the fastest kind of electrical communication between the neurons in your brain. As you start feeling drowsy, alpha waves take over.
As you enter Stage 1 Sleep, theta waves begin to predominate, and there are even more theta waves in Stages 2 and 3. When you finally enter REM sleep, in Stage 4, most of the neurons in your brain are firing very slowly. In Stage 4, your neurons are primarily generating delta waves.
What are the Stages of Sleep?
What's going on in the four stages of sleep? The Cliff Notes version of what happens during the four stages of sleep is something like this:
1. NREM Stage 1 (Shallow Sleep)
This first stage is relatively simple to understand. “Bedtime” is when your body receives signals that it’s time to go to sleep, like the sun going down, your body temperature peaks, and melatonin is released. Your “sleep switch” activates, and your body is ready for sleep. [saper, 2010]
For the first five to ten minutes, you go through a transition period from wakefulness to sleep. You spend just 5% of your total sleep time in this stage, the shortest amount. Your brain is still producing lots of high-amplitude theta waves.
These theta waves are mostly generated in your frontal cortex, the "executive" center of your brain. Muscle tone is still present, and your breathing is regular. Once you settle in, your muscles relax, your heart rate declines, and your brain waves abate, preparing you for sleep.
If someone wakes from this stage of sleep, you might tell them you were still awake, even though you weren't. Psychic phenomena are common in NREM Stage 1 or light sleep. You might experience a hallucination of some kind. It could be visual (even with your eyes closed) or auditory (even if you are wearing ear plugs).
You can also have hallucinations involving taste or smell. Your muscles may twitch suddenly or jerk violently. You may have the sensation of falling or spinning around. Usually, you won't remember these sensations when you wake up.
What’s Your Chronotype?
Your body has a preferred chronotype or sleep time or chronotype. A considerable number of people “push through” NREM Stage 1 get a second wind and remain awake. Individuals with insomnia or other sleep issues need to listen to their body and follow these sleep time signals.
2. NREM Stage 2 (Light Sleep)
NREM Stage 2 is also known as light sleep. For the next 20 minutes, your heart rate and breathing slow down and become more regular. Additionally, your muscles relax more, and your body temperature continues to drop. You become less aware of your surroundings.
During this time, eye movements stop, and brain waves show a new pattern. Your brain begins to produce a kind of wave known as "sleep spindles," an indication that is processing what you have learned during the day and forming new memories. These K complexes or “sleep spindles” are the most distinct of all brain waves seen on EEG and indicate you’re moving into a deeper stage of sleep.
Usually, someone can wake you fairly easily, or you may wake up to loud noise during NREM Stage 2. While the first N2 sleep stage lasts 10-25 minutes, each following cycle lasts longer. The American Sleep Foundation estimates that most people spend about half of their sleep time in Stage 2.
Scientists have observed that N2 sleep is important for filing, storing, and retrieving memories . So, if you want to make sure you really learn that new skill or are able to recall facts on trivia night, you want to make sure you get enough N2 sleep!
Fast Fact: Ever wake yourself up grinding your teeth? This is called “bruxism,” and it most often happens during NREM Stage 2 sleep. 
3. NREM Stage 3 (Deep Sleep)
In NREM Stage 3, your brain produces deep, slow delta waves, so many that this stage is known as delta sleep. During this stage of sleep, you probably won't be responsive to loud noises or changes in temperature.
It will be hard for someone to wake you up. Your brain produces growth hormone when you are in Stage 3 sleep. This is the hormone that stimulates the building of muscles and the burning of fat.
Because your body produces less growth hormone as you get older, not getting enough deep sleep causes weight gain. Your body also uses this stage of sleep to bolster your immune system. NREM Stage 3 is also the time your brain forms declarative memories.
This is the stage you need to be able to "sleep on" facts you have tried to memorize the day before, so you will remember them when you wake up.
Stage 3 sleep lasts 20 to 40 minutes.
4. REM Stage 4
In Rapid Eye Movement (REM) or Stage 4, your eyes move rapidly behind your closed eyelids like you are looking at things in a dream world. During this stage, your body becomes immobilized while your brain becomes almost as active as when you are awake.. Fortunately, assuming you don't suffer from a disorder of arousal, like sleepwalking, your muscles are paralyzed, so you can't act out your dreams.
What You Can Do to Improve Your Stages of Sleep
There is a long list of specific conditions that cause some people to lose sleep:
- Sleeping next to a snoring bed partner or being the person who does the snoring. Obstructive sleep apnea interrupts sleep dozens or even hundreds of times every night for the person who has it and anyone who lives with it.
- The need to get up to urinate at night.
- Chronic pain, like arthritis and fibromyalgia.
- Restless leg syndrome.
- Anxiety, depression, and bipolar disorder.
- Chronic health conditions like asthma, obesity, Alzheimer's, Parkinson's, and heart disease.
- Excessive use of or addiction to stimulants such as caffeine and nicotine.
The answers to these conditions are usually something you need to work out with your doctor.
Fortunately, not everyone has to deal with these impediments to a good night's sleep.
However, the temperature is something that affects everyone's sleep, whether it’s sleeping cold or enjoying a warm cozy bed temperature. Keep in mind that people fall asleep, stay asleep, and feel rested based on their spectrum of different temperatures.
Sleeping in a Warm Bed
When your bed is too warm, it's hard to fall asleep. You may stay awake until you are totally exhausted. Then you can finally fall asleep, complete Stage 1 sleep, and move into Stages 2 and 3. But you will have problems again at Stage 4.
Sleeping in a bed that is too hot can make it harder for your body to lower your core temperature. This keeps you from entering Stage 4 REM sleep. That's the stage when you can dream and process how you feel about your day.
How A Sleep Recipe Changed My Life
Physicist and Sleep Scientist Tara Youngblood teaches us how to optimize our waking hours by creating a sleep recipe we all need to recover and restore overnight. Watch her TEDx Talk as she discusses her recipe to sleep better.
Sleeping in a Cold Bed
In a cooler bed, you sleep longer because your brain can go through Stage 1 in just the usual five to ten minutes. You also enjoy better REM sleep because your body can slow down so your brain can speed up. You wake up emotionally as well as physically refreshed.
So, both NREM AND REM play a role in your overall sleep. Cycling through each sleep stage through the night is necessary for your physical and mental well-being. What behaviors and environmental factors can you tweak to support your body’s sleep process?
 Van Cauter, E., & Plat, L. (1996). Physiology of growth hormone secretion during sleep. The Journal of pediatrics, 128(5 Pt 2), S32–S37. View Study
 An Approach for Clinical Pulmonology; Editors. View Resource
 Effects of thermal environment on sleep and circadian rhythm. View Resource