During non-REM we cycle through 3 different phases of sleep before we enter our period of REM. Each stage of NREM sleep has a unique function and purpose. Below is a very brief overview of the 3 stages of NREM sleep.
Stage 1: The lightest stage of sleep. Sleeper is sensitive to external stimuli and can easily be awakened or aroused. Eye movements, muscle tone, and brain wave activity is gradually slowing down from the waking state. This is the state in which we experience hypnic jerks, muscle spasms, and feelings of falling. Sometimes you can wake from this state with no recollection of being asleep.
Stage 2: Eye movements cease, body temperature drops, and heart rate begins to slow. Brain activity is slow, mixed with bursts of high-frequency brain activation known as sleep spindles.
Stage 3: Also known as slow-wave sleep (SWS), is the deepest and most restorative stage of sleep. It is quite difficult to be awakened or aroused during this state. Muscle tone is relaxed but not paralyzed like REM sleep. It is for this reason that sleep disorders (parasomnias) such as sleep walking, sleep talking, and night terrors, commonly occur during this stage.
It is less likely that you’ll dream during NREM, however, it isn't impossible. Studies have shown that during NREM our mindset is thought-like and organized, where as during REM it is hallucinatory, incongruous, and bizarre. If dreaming does occur during NREM, it will likely occur during the period of deep sleep (stage 3). Dreams are reported to be less vivid during this state.
If we compare our brain activity during the lucid dreaming state and the NREM state you will see they are quite similar, however, when lucid, our brain activation is at its peak.
The NREM sleep state matures relatively late in comparison to REM sleep. Although it is unknown exactly when or how these states develop in early utero, studies suggest that we’re primarily in REM up until 26 weeks, when NREM then introduces itself and progressively increases for the remainder of our lives (NREM increases as REM decreases). This, therefore, suggests NREM doesn’t fully begin until we have enough brain to support it.
On average we cycle through the four stages of sleep four to five times a night, each cycle lasting approximately 90-120 mins long.
Want to learn about the fourth and final stage of our sleep cycle: the stage in which we harvest our dreams? Click here to read all about REM sleep!
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