There are two types of sleep stages, NREM and REM. These types of sleep occur in four stages.
Here's how that works:
As you sleep, your brain journeys through four stages of sleep of two different types.
Each stage of sleep plays a unique role in maintaining brain health and performance. Some stages power that ability of your brain and the rest of your body to make needed repairs and maintenance. You go through all four stages of sleep in a repeating cycle every night. Each repetition of the sleep cycle lasts longer and takes you into deeper sleep.
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.
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:
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'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:
For the first five to ten minutes, you go through a transition period from wakefulness to sleep. In Stage 1, 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. 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 this stage of sleep.
You might experience a hallucination of some kind. It could be visual even with your eyes closed), or my likely 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.
For the next 20 minutes, your body temperature drops, Your heart begins to slow down. Both your pulse and your breathing become more regular. During Stage 2 sleep, you become less aware of your surroundings. 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.
The American Sleep Foundation estimates that most people spend about half of their sleep time in Stage 2. This stage helps the brain slow down to enable the body to repair itself in NREM Stage 3.
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.
Stage 4 is rapid eye movement sleep. During this stage, your body becomes immobilized while your brain becomes almost as active as when you are awake. Your eyes move rapidly, even though your eyelids are closed as if you were seeing things in a dream world. 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.
Your brain processes emotions in Stage 4 sleep. It assembles emotional memories in Stage 4 the same way it assembles factual memories in Stage 3. The first period of REM sleep lasts about 10 minutes, but your dream time gets longer and longer throughout the night.
You don't progress through these two types of sleep stages in an exact order. You begin sleep in NREM Stage 1. NREM Stage 2 follows NREM Stage 1, and then NREM Stage 3 follows NREM Stage 2. But you go back into NREM Stage 2, leaving the deepest stage of sleep, before you enter REM sleep, Stage 4, when you can dream.
After REM sleep is over, your brain returns to NREM Stage 2 to start the cycle all over again, 1-2-3-2-4-2-3-2-4 and so on. You will go through your entire cycle four or five times during eight hours of sleep. If you were to spend the night hooked up to an EEG, it would record your sleep architecture, the exact stages of sleep you experience during the night and how long you experience them, in a recording called a hypnogram.
There is a long list of specific conditions that cause some people to lose sleep:
The answers for 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.
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 and 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.
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.
Essentials of Sleep Medicine