Stage 1 (NREM): This is the transition between wakefulness and sleep, which lasts only a few minutes. It is a stage of light sleep, and it is easy to wake up during this time.

Stage 2 (NREM): during this stage, your body temperature drops, your heart rate slows, and your brain begins to produce sleep spindles, which are brief bursts of activity.

Stages 3 and 4 (NREM): Known as deep sleep or slow wave sleep, these stages are vital for physical recovery. It’s harder to get up during these times, and when you do, you often feel groggy or disoriented.

Stage 5 (REM): REM sleep is when you are most dreamy, and your brain becomes most active. Your eyes move rapidly under your eyelids, and this stage is crucial for memory consolidation and learning.

When your body moves during this time, it goes through various physical and neurological changes that can sometimes lead to involuntary movements, such as shivering.

Hipnic Jerks: The most common cause of sudden twitching during sleep
The most frequent description of a sudden, involuntary twitching or twitching during sleep is what is known as a hypnic jerk, also called a sleep onset. This is a common condition, affecting nearly 70% of people at some point in their lives.