What are the 4 Stages of Sleep?

While asleep, a person’s body goes through multiple physiological changes as it moves from one stage to the next. These changes are part of the body’s natural sleep process, and completing all the stages is vital for physical and cognitive health.

Stage 1 (N1) is the first of the sleep cycles stages. Stage 1 occurs during the Non-REM sleep phase.

During Stage 1, the individual is between being awake and asleep. Therefore, Stage 1 is considered a light sleep, and the individual can awaken easily. Additionally, this stage is very short, lasting roughly 10 minutes.

The person’s heart rate, breathing, eye movements, and brain waves start slowing down at this time. The muscles also begin to relax, but you may experience some twitching as well.

Stage 2 (N2) is the second stage of sleep. Stage 2 occurs during the Non-REM sleep phase.

A person is still in a light sleep at this stage. However, Stage 2 is much longer, lasting about 30-60 minutes. During Stage 2, the muscles relax further, and the heart rate, breathing, eye movements, and brain waves continue to slow down.

The brain also starts to create sleep spindles, which are bursts of fast, brain wave activity. Sleep spindles are normally tracked on a electroencephalogram (EEG), which measures brain activity while a person is at rest.

Spindles can occur in different regions of the brain. Furthermore, researchers believe spindles may play an important part in sensory processing and memory consolidation.

Stage 3 (N3) is the third stage of sleep. Stage 3 is the final stage of the Non-REM sleep phase and the deepest stage of sleep.

Stage 3 is also known as ‘Slow Wave’ or ‘Deep Sleep’ because the brain waves are slowest during this time. The slow brain waves cause the individual to enter a deep sleep, and it can be harder to wake someone up in Stage 3.

Stage 3 lasts for roughly 20-40 minutes. During Stage 3, the muscles are relaxed, and the heart rate, breathing, eye movements, and brain activity hit their lowest levels.

During Stage 3, the body also releases vital hormones for functions like development and appetite. Stage 3 is also known as the restorative stage because it is when the body repairs tissues and replenishes its energy.

Stage 4 is REM Sleep. REM is the final part of the 4 stages of sleep and the end of the full sleep cycle.

REM usually occurs around 90 minutes into a sleep cycle. During Stage 4, the brain waves speed up, and the eyes quickly move from side to side. Brain activity is at its highest during the REM stage.

Stage 4 is when people dream, a byproduct of heightened brain activity. However, the muscles are in a state of paralysis to prevent people from physically acting out those dreams.

The importance of the stages of sleep is that each one is essential for reaping the physical and cognitive health benefits of sleep.

Stages 1 and 2 are important because they help prepare you for Stage 3 – or deep sleep. Stage 3 is important because it’s when the body restores itself, allowing you to have enough physical energy for the next day.

Stage 3 – or deep sleep – is also essential for physical growth and the hormones that control stress, appetite, and blood glucose levels. As a result, insufficient deep sleep can deter your body’s ability to regulate these functions. For example, people who don’t get enough rest have an increased risk of obesity or type 2 diabetes.

Additionally, Stage 3 is considered vital because it helps boost the immune system to ward off diseases and infections.

While Stage 3 is important for physical health, Stage 4 is considered an essential part of cognitive health. REM sleep is believed to be the time when memories are formed. Furthermore, research has revealed that REM sleep deprivation is linked to an increased risk of Dementia.

Most importantly, though, people need to complete all these sleep stages to function to the best of their ability.

The factors that affect the stages of sleep are age, gender, light, stress and anxiety, sleep disorders, pain, jet lag and shift work, and sleep environment. We go into detail about these factors that affect sleep below.

1. Age: Sleep changes as we age. Infants and children typically have longer deep sleep stages because their bodies are experiencing a high amount of development. However, the amount of deep sleep declines as a person reaches adulthood. Elderly individuals usually have shorter and fewer episodes of deep sleep.
2. Gender: Gender can affect the sleep stages. For example, women undergoing puberty, menstruation, pregnancy, and menopause may experience sleep pattern alterations.
3. Light: Light can affect the sleep stages because it can prevent the onset of Stage 1. The body has a natural internal clock that regulates sleep, and one of its most significant influences is light. More specifically, when it’s daylight, that tells the body to wake up, and when it’s dark, the body prepares itself for bed by increasing melatonin production. Melatonin is a sleep-inducing hormone. However, exposure to light – either natural or blue light from tech devices – can delay the natural sleep onset process.
4. Stress and Anxiety: Stress and anxiety can also prevent the onset of Stage 1 because you cannot relax enough to doze off. Additionally, anxiety may affect REM sleep. People dream during REM, but those with anxiety may be more likely to experience disturbing dreams or nightmares that cause them to awaken.
5. Sleep Disorders: Sleep disorders can also affect the sleep stages. Some disorders, like Restless Legs Syndrome or Insomnia, may delay entering Stage 1. However, other disorders, like Sleep Apnea or REM Sleep Behavior Disorder, can cause the individual to wake up in the later stages of sleep, such as N3 or REM.
6. Pain: Chronic pain and discomfort could make it difficult for someone to fall asleep or stay asleep during the night. Even though the first two sleep stages are lighter, most people sleep through them. However, uncomfortable pain could cause someone to wake up at the beginning of a cycle, preventing them from resting continuously through the night.
7. Jet Lag and Shift Work: Jet lag is a sleep disorder that occurs when you travel across time zones quickly. Jet lag can throw off your internal clock, and as a result, you may feel overly tired or have insomnia.
   Shift work is a sleep issue that results from working overnight or on an inconsistent schedule. When someone works overnight, they go against the body’s natural circadian rhythm that responds to light and dark. As a result, many shift workers cannot get enough rest during the day and often become sleep-deprived. People who have jet lag or shift work may find it difficult to fall asleep or get enough sleep. Most experts say adults should complete 5-6 sleep cycles, equating to about 7-9 hours of rest. So while completing sleep cycles is important, getting enough of them is equally crucial.
8. Sleep Environment: Your sleep environment can also impair the sleep stages. For example, if your bedroom is too warm, this could make it harder for you to fall asleep or cause you to suddenly wake up mid-cycle. Therefore, your environment should be cool, dark, and quiet to promote better shuteye.

Sleep disturbances are disorders that impair a person’s ability to fall or stay asleep. Sleep disorders can be caused by different factors such as stress, lifestyle, physical composition, and neurological issues. We will further explain common sleep disorders below.

1. Insomnia: Insomnia is a sleep disorder in which a person has difficulty falling asleep or staying asleep. Additionally, individuals with insomnia may wake up earlier than they should. Insomnia can be caused by factors such as stress, pain, and bad sleep habits.
2. Sleep Apnea: Sleep Apnea is when a person’s throat muscles relax to the point that it causes periodic pauses in breathing while they sleep. When this happens, there’s a lack of oxygen to the brain, and eventually, the body wakes itself up to get air. There are three types of Sleep Apnea: Obstructive, Central, and Complex. Obstructive is the most common and results from the throat muscles relaxing. However, Central Sleep Apnea happens when the brain is not sending the right signals to the breathing muscles. Finally, Complex Apnea is when a person has both Central and Obstructive symptoms.
3. Restless Legs Syndrome: Restless Legs Syndrome (RLS) occurs when an individual has a persistent urge to move their legs. RLS symptoms are typically more prevalent at night when the individual tries to relax. There is no known cause for RLS. However, scientific experts say it may be hereditary or caused by a chemical imbalance in the brain.
4. Circadian Rhythm Disorders: A Circadian Rhythm Disorder is when a person’s internal clock does not coincide with their surroundings. Symptoms include trouble falling asleep, staying asleep, and waking up too early. The most common types of Circadian Rhythm Disorders are Delayed Sleep Phase, Advanced Sleep Phase, Jet Lag, Shift Work, Irregular Sleep-Wake Rhythm, and Non-24-Hour Sleep-Wake Syndrome.
5. REM Sleep Behavior Disorder: REM Sleep Behavior Disorder is when a person can physically act out their dreams. Normally, the body is immobile during the REM phase. However, the nerve pathways that inhibit muscle movement do not work when a person has this disorder. As a result, the individual can move their limbs while dreaming.
6. Narcolepsy: Narcolepsy is a sleep disorder in which a person experiences extreme daytime tiredness and sudden urges to fall asleep. The direct cause of Narcolepsy is not known, but people with this disorder often have low levels of the wakefulness-regulating chemical Hypocretin. Experts aren’t sure what causes those low levels but suggest it could be genetic.

These disorders are harmful because they affect a person’s ability to achieve a healthy quality and quantity of sleep. Poor rest is linked to multiple health repercussions like high blood pressure, diabetes, heart attack, heart failure, and stroke. Furthermore, sleep deprivation hinders cognitive abilities and may lead to long-term memory problems such as Alzheimer’s.

Sleep disturbances affect sleep stages by diminishing sleep quality and sleep efficiency. Sleep quality refers to how restful your slumber is, while sleep efficiency measures the ratio of total time asleep to time in bed.

To achieve optimal rest, you need both good sleep quality and efficiency. You should finish all four sleep cycle stages to get good sleep quality. For example, if you wake up in the middle of the night, this hinders the quality of your rest.

On the other hand, good sleep efficiency means that you’re completing enough sleep cycles – or hours of sleep – overnight. Adults should rest for 5-6 cycles, equating to 7-9 hours.

When you don’t get good sleep quality and efficiency, this negatively affects your mental and physical health. You can experience symptoms that include worsening moods, trouble concentrating, and less physical energy.

The optimal length of sleep stages should fall between the average durations given by sleep health experts. You should base how much sleep you need according to these results.

Stage 1 should last around 10 minutes. Stage 2 should last between 30 and 60 minutes, and Stage 3 should last between 20 and 40 minutes. 

The first round of Stage 4 (REM Sleep) should last about 10 minutes, but each additional REM stage gets longer throughout the night.

The events of the sleep stages are the physiological changes that occur during each stage. The sleep stage events are listed below.

1. Dreaming: Dreaming occurs during the REM stage. Dreams are a series of emotions, thoughts, and images that involuntarily happen while asleep.
2. Faster Eye Movements: Fast eye movements occur during the REM stage. REM stands for the rapid eye movements that are indicative of this stage. During REM, the eyes will quickly move from side to side.
3. Slow Eye Movements: Slow eye movements occur during Stages 1, 2, and 3 of Non-REM sleep. Non-REM stands for the non-rapid eye movements that happen during this stage. Eye movement begins to slow during Stage 1 and continues to become slower into Stage 3 before picking back up again in Stage 4.
4. Faster Brain Waves: Faster brain waves occur during REM sleep. These quick brain waves are similar to when a person is fully awake. The increased brain activity is believed to be the cause behind dreams.
5. Slower Brain Waves: Slower brain waves occur during Stages 1-3 of Non-REM sleep. The brain waves begin to slow in N1 and continue to slow down through N3. Then, in Stage 4, they speed up again.
6. Muscles Relax: The muscles start to relax and continue to relax further throughout the sleep stages. When a person reaches REM, the muscles should be paralyzed enough to prevent them from physically acting out dreams.
7. Faster Breathing: Faster breathing occurs during REM sleep. Additionally, your breathing will become more irregular at this time.
8. Slower Breathing: Slower breathing occurs during Stages 1-3 of Non-REM sleep. Breathing begins to slow during Stage 1 and continues to stagnate through Stage 3 until it speeds up again in Stage 4.
9. Faster Heart Rate: A faster heart rate occurs during REM sleep. While the heartbeat speeds up, it shouldn’t become quicker than your normal heartbeat while awake.
10. Slower Heart Rate: A slower heart rate occurs during Stages 1, 2, and 3 of Non-REM sleep. The heart rate starts to slow during Stage 1 and continues to decline through Stage 3 until it picks back up in Stage 4.

You dream during Stage 4 – or REM sleep. During REM, brain activity increases. The increased brain activity is what causes dreams to appear. Additionally, your eyes will move quickly, your heart rate and breathing will speed up, and the muscles will become immobile.

Sleep spindles appear in Stage 2 of Non-REM sleep. While they may be present in the other two Non-REM stages, they are most prevalent in N2.

The sleep spindles definition is that they are sudden bursts of brain activity visible on an electroencephalography (EEG) test. EEG tests measure brain activity while someone sleeps. Studies have shown that spindles may be important for forming memories.

Body temperature changes according to the stages of sleep by decreasing before your bedtime and continuing to drop while you sleep. Then, body temperature starts to pick up when the morning arrives and continues to rise throughout the day. These sleep effects on the body are part of your natural circadian rhythm.

However, if a person’s core temperature is too high, this can hinder sleep onset and decrease time spent in deep sleep and REM sleep.

The tips to get quality sleep stages are following good sleep habits, having a comfortable sleep environment, and addressing sleep disorders. We share more details on these tips for better sleep below.

1. Good Sleep Habits: To improve your rest, you should follow good sleep habits. For example, stick to a sleep schedule every day, including weekends. Keeping a consistent bedtime and wake-up time helps train your body to feel tired and awake at the appropriate times. Another habit is to avoid daytime naps because these can make it harder for you to doze off at night. Third, you should avoid electronics before bed. Devices like smartphones, TVs, and computers emit a blue light that can delay sleep onset by reducing melatonin production. Melatonin is an important hormone that the brain releases to promote sleep. Fourth, try to manage stress and help yourself feel sleepy by establishing a relaxing nightly routine, such as taking a warm bath, meditating, or reading a book. Lastly, you should exercise regularly and avoid eating too much late at night. Exercise helps release endorphins to manage stress, which can help you rest better. If you eat too much food before bed, this can cause you to feel uncomfortable. Instead, reach for a light snack to curb any late-night hunger.
2. Comfortable Sleep Environment: A comfortable sleep environment should also improve your slumber. A good sleep space includes a quality bed and a cool, dark, and quiet bedroom. If you’re experiencing pain or discomfort, a well-made mattress can help alleviate these issues by providing both pressure relief and support. Additionally, people have been found to sleep better when the room is cool, dark, and quiet. Therefore, if you’re having trouble with your sleep quality and duration, you should make sure your mattress and bedroom support this.
3. Address Sleep Disorders: Addressing sleep disorders is also important for improving rest. In some cases, good sleep habits and a better environment can help. However, if you are still experiencing troubles, you should talk with your doctor. A physician can establish a more helpful form of treatment. For example, if you’re experiencing Sleep Apnea, your doctor may recommend CPAP, a popular form of treatment for this disorder.