What Is NREM Sleep?

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NREM Sleep

NREM, or non-REM (rapid eye movement), sleep encompasses the first three stages of sleep. It is characterized by thought-like mental activity and little to no eye movement.

Sleep is divided into four stages. The first three stages are part of NREM sleep, and the fourth stage is REM sleep, during which rapid eye movement occurs.

Approximately 75% to 80% of sleep is spent in NREM sleep. The first full NREM-REM sleep cycle is typically shorter, lasting around 70 to 100 minutes. Subsequent cycles last around 90 to 120 minutes. Most people go through four to six sleep cycles each night.

Signs of NREM Sleep

The body goes through many changes throughout NREM sleep. In general, NREM sleep is characterized by a period of slowing brain waves, heartbeat, and muscle activity. Some common signs associated with NREM sleep include:

  • Decreased heart rate
  • Decreased muscle activity
  • Lessened responses to outside stimuli
  • Muscle twitches and hypnic (myoclonic) jerks
  • Reduced body temperature
  • Release of growth hormones
  • Slowed breathing
  • Slowed brain waves
  • Spikes in brain activity include sleep spindles and k-complexes

While the three stages that occur during non-REM sleep share common features, each includes distinct events and characteristics.

Types of NREM Sleep

There are three stages of NREM sleep. Each stage of NREM sleep serves a unique purpose and plays an important role in supporting brain health. 

NREM Stage 1

The first stage of NREM sleep acts as a transition between wakefulness and the deeper stages of sleep. During stage 1, brain waves begin to slow. Breathing, heartbeat, and body movements are also slow. While muscles become more relaxed, you may also experience periodic twitches or jerks.

Stage one is the briefest stage of sleep, lasting just five to 10 minutes. If you are awakened during stage 1, you may feel as if you weren't really asleep at all—which is why this stage is sometimes referred to as feel like a period of relaxed wakefulness. While activity is slowing at this point, the brain is still fairly active.

NREM Stage 2

The second stage of sleep is characterized by slowed body activity and decreased awareness. During this stage:

  • Body temperature decreases
  • Breathing and heart rate slow
  • Eye movements cease
  • Awareness of external stimuli declines

During stage two, the brain produces two distinct patterns of brain waves. Sleep spindles are short spikes in brain activity. K-complexes are sharp bursts of electrical activity followed by a brief dip in activity.

Both sleep spindles and k-complexes are believed to play an essential role in memory consolidation, which involves processing the memories and information people acquire during the day.

Stage 2 NREM sleep accounts for approximately 50% of all time spent sleeping.

Stage 3

The third stage of NREM sleep is also the deepest. It is marked by the emergence of large, low-frequency delta waves and even slower activity known as slow oscillations.

Stage 3 NREM sleep is marked by:

  • Decreased blood pressure and breathing rate
  • Complete muscle relaxation
  • Less responsiveness to external stimuli
  • Difficulty being wakened
  • Sleep inertia when woken, a period of severe grogginess

Stage 3 sleep is a deep sleep essential for consolidating certain types of memories, mainly declarative ones. These are memories of specific facts and events, such as things that you personally experienced and information that you have learned.

It is vital to get enough stage 3 NREM sleep each night. This type of sleep causes people to feel refreshed and well-rested the next day.

NREM Sleep Sequence

The stages of sleep are not perfectly sequential. During a sleep cycle, people go through stages 1, 2, and 3 of NREM sleep, and then stage 2 is repeated again before REM sleep begins. Following REM sleep, the NREM cycle begins again, starting with stage 2. Each cycle lasts around 90 to 110 minutes, and roughly four to five cycles may occur each night.

Impact of NREM Sleep

NREM sleep serves several essential physical and mental functions. In addition to aiding in memory consolidation, the body builds bones, repairs muscle, regenerates tissues, and boosts the immune system during this time, particularly during the period of deep sleep that occurs in stage 3.

Memory Consolidation

Memory consolidation occurs durion NREM sleep, which is critical to learning. This period of sleep has been shown to affect both procedural memory (which involves skills and the ability to learn new tasks) and declarative memory (which is the memory for facts and information).

Research has shown that specific brain patterns that occur during NREM lead to a variety of cognitive improvements, including better motor learning, word retrieval, working memory, and verbal fluency. 

Evidence also indicates that age-related problems with NREM sleep can play a part in memory problems that are associated with the aging process. Slow-wave sleep naturally declines with age, dropping more than 60% between the ages of 10 and 20.

Repair, Restoration, and Growth

NREM sleep is believed to play an essential role in physical growth and repair of body tissues. Research has shown, for example, that blood pressure drops during slow-wave sleep, which can play an important part in lowering the risk of cardiovascular disease.

People with sleep conditions that prevent these nightly drops in blood pressure, such as insomnia and sleep apnea, have a higher risk, for heart disease.

Consequences of Disrupted NREM Sleep

Interruptions to the normal sleep cycle can have a variety of mental and physical health effects. Some problems that may emerge if the sleep cycle is disrupted include:

Challenges Affecting NREM Sleep

Getting enough sleep each night is vital for short- and long-term health. Your sleep needs vary depending on your age.

According to the American Academy of Sleep Medicine, adults over 18 need a minimum of 7 hours of sleep per night. Sleep needs also tend to decline as people age, which is why children require more sleep than adults.

Unfortunately, many factors can interfere with NREM sleep, including:

  • Stress, anxiety, or depression
  • Caffeine, nicotine, and alcohol consumption
  • Work schedules that include night shifts or early morning hours
  • Certain medications, such as those for colds, allergies, and asthma
  • Medical conditions like sleep apnea, restless legs syndrome (RLS), and narcolepsy

Making sure you get enough sleep each night is crucial for maintaining your health. If you think you might be experiencing a sleep disorder, talk to your doctor. A diagnosis and treatment plan can help you get the restful sleep you need.

Recap

The bottom line is that NREM sleep is essential for mental and physical health. Disruptions to the normal sleep cycle can have a variety of negative consequences, so it's important to make sure you get enough restful sleep each night. If you think you might be suffering from a sleep disorder, talk to your doctor for help.

Tips for Improving NREM Sleep

Fortunately, there are steps you can take to protect your NREM sleep.

Stay Physically Active

Exercise is important for overall health, and it can also help improve the quality of your sleep. Just be sure to avoid working out too close to bedtime, as this might actually make it harder to fall asleep

Create a Relaxing Bedtime Routine

Preparing for sleep should be an opportunity to wind down and relax. A soothing bedtime routine can help signal to your body that it's time to sleep. This might include taking a warm bath, reading a book, or stretching. Yoga, for example, may help promote improved sleep quality.

Limit Caffeine and Alcohol Intake

Both caffeine and alcohol can disrupt sleep, so it's best to avoid them in the hours before bedtime. Caffeine can stay in your system for up to 8 hours, so it's especially important to limit intake in the afternoon and evening.

Create a Sleep-Friendly Environment

Your bedroom should be a dark, quiet, and cool place to sleep. This can help promote relaxation and make it easier to fall asleep.

Go to Bed and Wake at the Same Time

Establishing a regular sleep schedule can be helpful for people with insomnia. Going to bed and waking up at the same time each day can help regulate your body's natural sleep-wake cycle and make it easier to fall asleep and stay asleep.

Use Good Sleep Habits

In addition to going to bed at the same time each night, you should focus on creating a restful sleep environment and a before-bed routine that promotes relaxation. Following a routine each night can help signal to your brain and body that it is time to go to sleep.

Avoid Blue Light at Bedtime

The blue light emitted by screens on phones, computers, and TVs can disrupt the body's natural sleep-wake cycle. To avoid this, establish a cutoff time for screen use before bed and avoid using devices in the hours leading up to sleep.

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By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.