The Complete Guide to Sleep Cycle Stages

Sleep can be the most delicious or the most maddening thing. You may lay your head on the pillow with no hurry, no worries, and immediately drift off into blissful oblivion. If you are not so lucky, you may have a mind full of racing thoughts, an alarm that is set for way too soon, and hours of desperate, agitated wakefulness.

Scientists have not completely solved the mystery of sleep. They cannot tell you why you slept soundly last night, but you struggle to sleep at all tonight. What they have done is map the sleep cycles. Learning about these cycles can help you understand how you sleep, why sleep deprivation is harmful, and why it matters at what point you wake up.

Falling Asleep Is Not a Passive Process

You may not realize that you do not really just fall asleep. Your brain does not run down at the end of the day like a wind-up toy that has unwound. There is a process through which your brain actively transitions from wakefulness to sleep and back to wakefulness. This process is reflected in the sleep cycle stages. These stages are designated as follows.

• Non-REM sleep, Stage 1
• Non-REM sleep, Stage 2
• Non-REM sleep, Stage 3
• Non-REM sleep, Stage 4
• REM sleep

Non-REM Sleep

You spend most of your time sleeping in non-REM sleep. REM stands for rapid eye movement. This is the condition in which your eyes are literally twitching left and right beneath your eyelids. This movement only occurs during specific periods called REM sleep, so the rest of the time is simply called non-REM, or nREM.

There are four stages of nREM sleep: stage 1, stage 2, stage 3, and stage 4. When you first fall asleep, you are in stage 1. You then sleep progressively more deeply until you reach stage 4. Then you cycle back through stage 3, stage 2, and stage 1. This sequence repeats and continues to cycle until you awaken.

Your first trip from stage 1 through stage 4 and back to stage 1 will last about an hour and a half. Subsequent cycles last a bit longer, up to about two hours. These times vary from person to person, though, and even change as you age from infancy through advanced age.

Stage 1 of nREM Sleep

You enter stage 1 immediately upon falling asleep. This stage lasts about 10 minutes. The waves recorded from your brain as you pass from wakefulness into sleep are primarily alpha waves. Theta waves begin to appear and predominate as you become truly asleep. Theta waves occur at a rate of 4 to 7 cycles per second.

When you are in stage 1 sleep, you can be awakened very easily. Most people will not be disoriented when they are awakened from stage 1. You still have muscle tone during this stage since your muscles have not completely relaxed yet.

Think of a time when you dozed off for a minute or two because you were tired or bored, and you will understand how light stage 1 is and how easily you can wake from it.

You have also probably experienced hypnagogic jerks during stage 1. This happens when you are right on the edge of sleep, and your muscles suddenly jerk. This is a harmless phenomenon, although it can be unnerving when it is paired with a feeling of falling.

Stage 2 of nREM Sleep

Stage 2 is still a relatively light sleep phase, but it is beginning to deepen from stage 1. Your heartbeat becomes slower. Your body temperature becomes lower. You begin to lose muscle tone, and you will be more difficult to awaken.

During stage 2, your brain will begin to produce waves known as sleep spindles. There are a couple of different types of spindles, and they are recorded from specific areas of the brain. Scientists are not sure yet what these waves indicate about what the brain is doing at that time.

Waves called K-Complexes are also produced during stage 2. These are single waves that last for just a second. Sometimes they occur spontaneously, but they can also be caused by a brief noise, like a knock or rattle. The purpose of K-Complexes and sleep spindles are mysteries yet to be solved.

Stage 2 will begin to transition to stage 3 about 20 minutes after you fall asleep. If you want to take a short nap, keeping it under 20 minutes is ideal. This keeps you from drifting into the deeper phases of sleep, and you will be able to awaken somewhat refreshed without feeling groggy and disoriented.

Stage 3 and Stage 4 of nREM Sleep

Some sleep specialists do not distinguish between these phases anymore; they just combine them into a single stage 3. These phases tend to fade into each other and are not sharply distinct.

Stage 3 and stage 4 are the deepest levels of sleep. As you move from stage 2 into stage 3, your brain begins to produce long, slow, delta waves. These predominate in stage 4 and have a frequency of less than 4 cycles per second. Stages 3 and 4 are sometimes referred to as slow wave sleep, or SWS.

Researchers used to think that the brain was inactive, essentially resting, during these stages of sleep. They now know that this is not true at all. Your brain and body are actually quite busy during this time. You can think of it as a time when your brain turns off the phone and gets stuff done around the house. 

According to Harvard Medical School, the following functions are a few of the things your brain and body take care of during sleep stages 3 and 4.

• Repairing the tissues of the body.
• Engaging in growth, especially in children.
• Building bone.
• Bolstering the immune system.
• Synthesizing proteins.
• Clearing adenosine from the brain.
• Processing some types of visual learning.

You will be difficult to awaken during these deepest stages of sleep. Your heart rate is at its lowest, and you have little or no muscle tone. You do not wake up naturally out of stage 4, and when you are jarred awake by an alarm or other stimulus, you will often exhibit what is called sleep inertia.

Sleep inertia may be a feeling that is all too familiar to you. You are disoriented and unfocused. You may feel as though your brain has not “turned on” for up to 30 minutes after you have been awakened. It is best not to attempt any complex tasks during this time since your cognitive functioning may be impaired.

After a period of sleep in stage 4, you will go back through stage 3, stage 2, and stage 1. You will probably have the longest stage 4 the first time you go through this cycle. Each time the cycle repeats, the time you spend in stage 4 will be shorter. This surprises some people. It seems natural to think that you will sleep more deeply as the night goes on, but that is not the way it works.

Most dreaming takes place during REM sleep, but to the extent that it occurs in the nREM phases, it occurs in sleep cycle stages 3 and 4. Your dreams in these stages are usually rather ordinary, and they may reflect events that occurred during the day.

REM Sleep

Rapid eye movement, or REM sleep, is the phase during which most of your dreams occur. When your sleep cycle has moved through stage 4 and back up to stage 1 the first time, you may have your first period of REM sleep. Each time you cycle back to stage 1 during the night, you may have another session of REM sleep. The dreams you have during REM sleep are likely to be the odd ones that you laugh about or try to find hidden meaning in the next day.

Your brain exhibits a mixed pattern of brain waves during REM sleep. It closely resembles a normal waking pattern. Your brain is also consuming a great deal of energy during this phase, which indicates a high activity level. Sometimes it appears to be even more active than when you are awake. Your heart rate increases, your blood pressure increases, and your breathing becomes irregular.

The duration of your REM phases is the opposite of the duration of your stage 4 periods. You spend less time in stage 4 as the night wears on but more time in REM sleep. Your first REM session may only last 10 minutes, but the last one may continue for an hour.

REM sleep is like stage 1 in that you are sleeping lightly and can be easily awakened. However, during REM sleep, unlike stage 1, you have almost no muscle tone. You are literally paralyzed during this phase. Some sleep specialists believe this is a protective measure. It keeps you from acting out your dreams and possibly hurting yourself.

It is common for people to wake up for just a few seconds in each REM phase. You will not remember these awakenings. If you wake up fully in this phase, though, you will often remember the dream you were having. Some people believe they do not dream, but if they participate in a sleep study and are awakened by the researchers during REM sleep, they will be surprised to find that they were dreaming.

Researchers have still not determined the purpose of dreams. They do know that your brain consolidates memories and processes information during REM sleep, but that does not explain why you have the often bizarre movies in your head while this is happening.

Changes in Sleep Cycle Stages With Age

The time you spend in each stage of sleep changes as you move through your life. As an infant, you may have spent up to 50% of your sleep cycle in REM sleep. As an adult, that has likely dropped to 20%.

This makes sense if you remember that your brain works on memories and processing during REM sleep. Infants and young children have a great deal of work to do to figure out the world, and their brains need extra time to put it all together.

As you become elderly, the time you spend in the deep stages of 3 and 4 tends to decrease, and you spend more time in stage 2. Although older people often sleep for shorter periods of time than when they were younger, they still require the same total amount of sleep. They may need to take naps during the day to make up for shorter sleep periods at night.

The Effects of Sleep Deprivation

When you are specifically deprived of REM sleep, the result is quite noticeable. At the first opportunity, you engage in what is called rebound sleep. This means that your brain does not wait to cycle through stage 4 and back up to stage 1 before it goes into REM. It phases into REM almost immediately and stays in this stage longer than usual.

Sometimes people are unable to achieve REM sleep because they are taking medication that inhibits it. In these cases, they dream more than is typical in nREM sleep.

If any stage of your nREM sleep is repeatedly disrupted, it impairs your ability to function normally. You feel “foggy” and find it difficult to perform normal daily tasks. You may also be irritable and forgetful. You exhibit poor judgement and make poor decisions. You will find it difficult to learn new information, and you may not be able to access information you already know.

Sleep deprivation can also have serious effects on your physical health. According to Harvard Medical School, it has been linked to cardiovascular disease and diabetes. It puts you at higher risk for obesity. Your impaired ability to focus and judge the situation may make you more susceptible to having a serious or fatal accident.

Your brain will attempt to make up for nREM sleep when it gets the chance, but it does not interrupt the normal pattern of the sleep cycle to do it. You may just spend more time in the stage that was interrupted when you naturally reach that stage. The stages of sleep seem to be “hardwired” in, and as soon as you are allowed to sleep as you wish, you will fall back into your normal patterns.