Understanding Sleep Cycles: Nrem And Rem Phases Explained

Sleep is a complex and mysterious body process that is essential for the body and brain to rest and recover. On average, a good night's sleep consists of four to six sleep cycles, each lasting between 90 and 120 minutes. Each cycle is further divided into four stages, three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage.

The four stages of sleep

Sleep is a complex and mysterious body process that is essential for humans to function. It is divided into four distinct stages, each with its unique characteristics and functions. These stages are categorised into two types: non-rapid eye movement (NREM) sleep, which consists of the first three stages, and rapid eye movement (REM) sleep, which is the fourth stage.

Stage 1 (NREM)

Stage 1 is the lightest stage of sleep and usually lasts only a few minutes. During this stage, the body transitions from wakefulness to sleep, with a slowdown in heart rate, breathing, eye movements, and brain waves. The muscles relax, but occasional twitching may occur.

Stage 2 (NREM)

Stage 2 is a deeper level of sleep where the body temperature drops, eye movements stop, and breathing and heart rate become more regular. The brain also produces bursts of rapid brain wave activity called sleep spindles, which are associated with memory consolidation. This stage accounts for about 45-50% of total sleep time.

Stage 3 (NREM)

Stage 3 is the deepest and hardest stage of sleep to wake up from. It is characterised by slow and strong brain waves, and the body further relaxes with decreased muscle tone, blood pressure, and breathing. This stage is crucial for physical repair, immune system strengthening, and memory consolidation. Adults spend about 25% of their total sleep time in this stage.

Stage 4 (REM)

The final stage is REM sleep, known for rapid eye movements and dreams. Breathing becomes faster and irregular, while blood pressure and heart rate may become less stable. REM sleep is important for learning and memory, and it is when the brain repairs itself and processes emotions and experiences. This stage makes up about 25% of total sleep time in adults.

Cycling Through Sleep Stages

Throughout the night, the body cycles through these four stages multiple times, with each cycle lasting around 90 to 120 minutes. The progression of stages is not always linear, and factors such as age, sleep patterns, and health conditions can influence the duration and quality of each stage.

Understanding the sleep cycle is vital as it highlights the importance of adequate sleep for physical and mental well-being. Disruptions to the sleep cycle can have short-term and long-term consequences, impacting various aspects of daily life.

Non-REM sleep

The first stage of non-REM sleep is a transitional period between wakefulness and sleep. During this stage, your brain slows down, and your heartbeat, eye movements, and breathing slow with it. Your body relaxes, and your muscles may twitch. This stage lasts for around five to ten minutes, and it is easy to wake someone up during this stage.

The second stage of non-REM sleep is when you enter light sleep. Your body temperature drops, your eye movements stop, and your breathing and heart rate become more regular. Your brain also begins to produce bursts of rapid, rhythmic brain wave activity, known as sleep spindles, which are thought to be a feature of memory consolidation. This stage lasts for about 10 to 25 minutes and accounts for about half of your total sleep time.

The third and fourth stages of non-REM sleep are collectively referred to as slow-wave sleep (SWS) and are the deep sleep stages. During these stages, your muscles are completely relaxed, your blood pressure drops, and your breathing slows down even further. This is the most challenging stage to wake someone up from, and if they do wake up, they may feel disoriented for a few minutes. Slow-wave sleep is critical for restorative sleep, allowing for bodily recovery, growth, and strengthening the immune system. The amount of time spent in slow-wave sleep decreases as the night progresses, with longer periods in the early sleep cycles and shorter periods in the later cycles.

Overall, non-REM sleep is essential for the body to repair and regenerate tissues, build bone and muscle, and strengthen the immune system. It also plays a crucial role in memory consolidation and preparing the brain for the next day. The progression of non-REM sleep stages ensures that the body and mind are well-rested and prepared for optimal functioning.

REM sleep

After falling asleep, the body first enters non-REM sleep, followed by a shorter period of REM sleep, after which the cycle starts over. Each cycle lasts between 90 and 120 minutes, and if you get eight hours of sleep each night, you will usually go through four or five cycles.

The first REM stage typically lasts around 10 minutes, with each subsequent stage getting longer. The final REM stage may last up to an hour. The body enters a state of paralysis during REM sleep, which prevents you from acting out your dreams.

The amount of time spent in REM sleep changes throughout the night and over a person's lifetime. Babies spend up to 50% of their sleep in REM, while adults spend only about 20%. As people age, they tend to sleep more lightly and get less deep sleep, and older adults tend to spend less time in REM sleep.

Sleep cycles and stages

Sleep is a complex and mysterious body process, and modern medicine is still working to fully understand it. However, we do know that sleep cycles and stages play a crucial role in ensuring we get the rest we need.

When we sleep, we cycle between non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Each cycle includes three stages of NREM sleep followed by a stage of REM sleep. The first stage of NREM sleep is a transition period between wakefulness and sleep, where the body starts to slow down. This is followed by a deeper stage of NREM sleep, where the body temperature drops and breathing and heart rate become more regular. The third stage of NREM sleep is the deep sleep stage, which is harder to wake up from and is when the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. After the third stage of NREM sleep, we move into the REM stage, where the eyes move rapidly and dreams typically occur. REM sleep stimulates the brain regions associated with learning and memory and is important for processing emotions and storing emotional memories.

Each sleep cycle typically lasts between 90 and 120 minutes, and we go through four to six cycles per night. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes, while later cycles tend to be longer, falling between 90 and 120 minutes. The amount of time spent in each stage of sleep also changes throughout the night. For example, we spend more time in REM sleep in the second half of the night, and the REM stages get longer as the night progresses.

As we age, our sleep patterns change. Newborns, for instance, fall asleep through REM sleep and spend a much greater portion of the night in this stage compared to adults. As we get older, we tend to spend less time in REM sleep and deep sleep, and sleep becomes lighter and less consolidated.

Understanding sleep cycles and stages is crucial because it helps us recognise the importance of getting sufficient, quality sleep. Not getting enough sleep can have short-term effects such as trouble concentrating, mood changes, and fatigue, as well as long-term health impacts like cardiovascular disease and cognitive impairment. Therefore, it is essential to prioritise sleep and maintain healthy sleep habits to ensure we get the restorative sleep we need.

Sleep disorders

• Central disorders of hypersomnolence: These include narcolepsy, which often causes people to enter REM sleep directly, skipping the first three stages.
• Circadian rhythm sleep-wake disorders: Examples include jet lag and shift work sleep disorder.
• Sleep-disordered breathing: Obstructive sleep apnea can interrupt the healthy sleep cycle and reduce the amount of deep sleep and REM sleep due to breathing difficulties.
• Sleep-related movement disorders: Restless leg syndrome falls under this category.
• Parasomnias: These disorders can affect either NREM or REM sleep, or both. Examples include sleep myoclonus, sleepwalking, nightmares, and REM sleep behaviour disorder.
• Other disorders: Other conditions that may affect sleep include insomnia, depression, anxiety, and Alzheimer's disease.

Age-related changes in sleep architecture can also contribute to sleep disorders. For example, older adults tend to experience earlier bedtimes and wake-up times, shorter sleep episodes, and reduced sleep efficiency. They also spend less time in the deeper stages of NREM sleep and REM sleep.

Additionally, certain medications, substances, and lifestyle factors can negatively impact sleep quality. These include caffeine, alcohol, nicotine, and electronic screens, which should be avoided close to bedtime.

Frequently asked questions