Sleep Stages: Understanding The Pre-Rem Phase

Sleep is a complex and mysterious process that is essential for the human body and brain to rest and recover. During sleep, the body cycles between non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further divided into three stages, with each stage leading to progressively deeper sleep. After falling asleep, individuals typically enter NREM stage 1, followed by stages 2 and 3, before transitioning into REM sleep. Thus, NREM sleep usually comes before REM sleep.

Non-rapid eye movement (NREM) sleep comes before REM sleep

Non-rapid eye movement (NREM) sleep is the stage of sleep that comes before rapid eye movement (REM) sleep. NREM sleep is often referred to as deep sleep, and it is characterised by slower brain waves and a more relaxed body. During this stage, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system.

NREM sleep is further divided into three substages: N1, N2, and N3. In the first stage, N1, individuals feel sleepy and can easily be awakened. This is a light sleep stage characterised by theta waves, slower brain waves that indicate a relaxed state. The N2 stage is the next stage of sleep, during which electrical brain waves increase and become what are known as theta waves. It is harder to wake someone during this stage, and sleep spindles and K-complexes—which help induce deep sleep—are found during this stage. The third and final stage of NREM sleep is N3, which is the deep sleep stage. This is the most difficult stage to wake someone from, and if they do wake, they will likely experience sleep inertia, a state of confusion or mental fog that can last up to 30 minutes.

After progressing through the three stages of NREM sleep, the body then enters REM sleep. REM sleep is characterised by rapid eye movements and more active brain activity. Dreams typically occur during this stage. Each cycle of sleep, which includes the three stages of NREM sleep and the REM stage, lasts between 90 and 120 minutes, and individuals will go through four to six cycles per night.

NREM sleep is further divided into three stages: N1, N2, and N3

N1

N1 is the lightest stage of sleep. It is the typical transition from wakefulness to sleep and generally only lasts a few minutes. During this stage, eye movements are slow and rolling, and the heartbeat and breathing slow down as muscles begin to relax.

N2

N2 is a deeper stage of sleep, from which sleepers can still be easily awakened. It is marked by a further slowing of the heartbeat and breathing, and a drop in body temperature. Two distinct brain wave features appear for the first time: sleep spindles and K-complexes. Sleep spindles are brief, powerful bursts of neuronal firing that are believed to be integral to synaptic plasticity and memory consolidation. K-complexes are long delta waves that last approximately one second and are thought to function in maintaining sleep and memory consolidation.

N3

N3 is the deepest stage of sleep and is also known as slow-wave, or delta, sleep. It is the most difficult stage to be awakened from, and if someone is awoken during this stage, they will likely experience sleep inertia, a state of confusion or mental fog that can last for up to an hour. During N3, the body performs a variety of important health-promoting functions, including tissue repair and growth, cell regeneration, and strengthening the immune system.

N1 is the lightest stage of sleep, lasting only a few minutes

During N1, the body has not yet fully relaxed, but bodily functions start to slow down, and there are brief movements and light brain activity. As the night progresses, a sleeper who is uninterrupted will not spend much time in this stage as they move through further sleep cycles.

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 first REM cycle of the night typically lasts around 10 minutes, with each subsequent cycle getting longer, up to an hour for the final cycle.

N1 is the first of four stages of sleep, which are divided into three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage. After N1 comes N2, then N3, and finally REM sleep.

N2 is deeper sleep, where the body prepares for N3 and REM sleep

N2 is the second stage of non-rapid eye movement (NREM) sleep, and it is deeper than the first stage, N1. During this stage, the body prepares for the deep sleep of N3 and the rapid eye movement (REM) stage that follows.

In N2, the body enters a more subdued state, with a drop in temperature, relaxed muscles, and slower breathing and heart rate. Brain waves also show a new pattern, and eye movement stops. While brain activity slows, there are short bursts of activity that help the sleeper resist being woken up by external stimuli. This stage is also characterised by sleep spindles and K-complexes, which are essential for memory consolidation.

N2 usually lasts for 10 to 25 minutes during the first sleep cycle, and each successive N2 stage becomes longer. Collectively, a person typically spends about half of their sleep time in N2.

After N2, the body progresses into N3 sleep, which is even deeper and harder to be woken up from. This is followed by REM sleep, where the eyes move rapidly and dreaming occurs.

N3 is deep sleep, where the body repairs and restores itself

Sleep is a complex and mysterious process that is essential for the human body and brain to rest and recover. During sleep, the body cycles between being awake and asleep, with certain processes only occurring during sleep. One of the key functions of sleep is self-repair and recovery, where the body takes advantage of being less active to heal injuries and repair any issues that occurred while awake.

N3 is the deep sleep stage, also known as delta wave sleep or slow-wave sleep. It is the third stage of non-rapid eye movement (NREM) sleep, which is further divided into three substages: N1, N2, and N3. N3 typically occurs during the first third of the night's sleep. During this stage, the body repairs and restores itself, including the regeneration of neurons within the cerebral cortex and the formation of new memories. It is harder to wake someone up during N3 sleep, and if they do wake up, they may experience sleep inertia, a state of confusion or mental fog that can last for about 30 minutes.

N3 sleep is crucial for waking up feeling rested. Without sufficient N3 sleep, one might feel tired and drained, even after a long sleep. This is because the body automatically tries to get as much N3 sleep as possible early in the sleep cycle. N3 sleep is also important for the immune system, as chemicals that strengthen the immune system circulate in the blood during this stage.

As people age, the amount of N3 sleep they get decreases. Children and teenagers require more N3 sleep than adults, and by the time individuals reach 65, the amount of N3 sleep may decrease to zero. Therefore, it is important to prioritize getting adequate sleep, including sufficient N3 sleep, to maintain overall health and well-being.

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