The sleep cycle consists of four stages of non-rapid eye movement (NREM) sleep and one stage of rapid eye movement (REM) sleep. Here is the overview of each cycle:
A).Stage 1 (NREM-1): This is the transition phase between wakefulness and sleep. It lasts only a few minutes, during which the body begins to relax, and brain activity starts to slow down. It’s common to experience sudden muscle contractions or a feeling of falling during this stage.
Key characteristics of NREM-1 are:
- Light Sleep: NREM-1 is considered a light sleep stage, and it typically lasts for a few minutes.
- Brain Activity: Brain waves in NREM-1 are slower than those during wakefulness but still faster than in later sleep stages. These brain waves are known as alpha and theta waves.
- Muscle Relaxation: Muscle activity begins to decrease during NREM-1, and some people might experience sudden muscle contractions known as hypnic myoclonia.
- Hypnic Jerks: Some individuals might experience hypnic jerks or the sensation of falling during this stage, which is a normal phenomenon.
B).Stage 2 (NREM-2): In this stage, the body continues to relax, and brain activity further slows down. Sleep spindles (short bursts of rapid brain waves) and K-complexes (single, large, slow waves) may occur during this phase.
Some Key characteristics of NREM-2 are:
- Brain Activity: Brain waves in NREM-2 are slower than in NREM-1, but they also include bursts of rapid brainwave activity known as sleep spindles and K-complexes. These neural activities are thought to play a role in memory consolidation and information processing.
- Muscle Relaxation: Muscles are more relaxed during NREM-2 compared to wakefulness, but not as deeply relaxed as during the deeper stages of NREM sleep.
- Eye Movement: Unlike REM sleep where rapid eye movements occur, eye movements in NREM-2 are slow and infrequent.
- Body Temperature and Heart Rate: Both body temperature and heart rate continue to decrease during this stage as the body settles further into sleep.
- Sleep Depth: It is still relatively easy to wake a person during NREM-2, as they are not in the deep sleep stages unlike (NREM-3).
C). Stage 3 (NREM-3): Also known as deep sleep or slow-wave sleep, this stage is crucial for physical restoration and growth. Brain waves become even slower, and it becomes harder to wake someone during this deep sleep.
Key characteristics of NREM-3 (deep sleep) include:
- Brain Activity: Brain waves during NREM-3 are the slowest and have the highest amplitude compared to other sleep stages. These slow-wave brain patterns are often referred to as delta waves.
- Muscle Relaxation: Muscles are profoundly relaxed during NREM-3, and it can be challenging to awaken someone from this stage of sleep.
- Limited Eye Movement: Eye movements are minimal or absent during deep sleep.
- Physical Restoration: NREM-3 is crucial for physical recovery and healing. Tissue repair, muscle growth, and the release of growth hormones occur during this stage.
- Memory Consolidation: Deep sleep is also involved in memory consolidation, helping to solidify memories and information learned during the day.
- Sleepwalking and Night Terrors: Sleepwalking and night terrors, which are types of sleep disorders, are more likely to occur during NREM-3.
D).Stage 4 (REM): Rapid Eye Movement (REM) sleep is characterized by increased brain activity, rapid eye movements, and vivid dreams. REM sleep is essential for memory consolidation and emotional processing. Most dreaming occurs during this stage
The sleep cycle typically progresses through stages of non-REM sleep (NREM-1, NREM-2, NREM-3) before entering REM sleep. After REM sleep, the cycle starts again with NREM-1. Each sleep cycle lasts approximately 90 to 110 minutes, and throughout the night, the duration of REM sleep tends to increase with each subsequent cycle.
Here are some key features of REM sleep:
- Brain Activity: During REM sleep, brain activity increases and becomes similar to that of wakefulness. However, voluntary muscle movement is inhibited, leading to temporary paralysis of the body (known as REM atonia).
- Rapid Eye Movements: As the name suggests, REM sleep is marked by rapid and random movements of the eyes. These eye movements are associated with dreaming activity.
- Vivid Dreams: REM sleep is when most dreaming occurs. Dreams during this stage can be highly vivid, emotional, and sometimes bizarre.
- Memory Consolidation: REM sleep plays a crucial role in memory consolidation and processing information from the day, contributing to learning and memory functions.
- Emotional Regulation: This stage of sleep is believed to be essential for emotional regulation and processing, helping to balance emotions and adapt to daily experiences.
- Physiological Changes: Heart rate and breathing may become irregular during REM sleep, and blood flow to the brain increases.