The beginning on non-REM sleep, also called NREM1. Characterized by small amplitude EEG waves of irregular frequency, slow heart rate, and reduced muscle tension. Alpha rhythms appear during relaxation. Sharp waves called vertex spikes appear. 

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A stage of sleep without rapid eye movements which divide into stages 1, 2, and 3 sleep (NREM 1, 2, 3).
- Stage 1
- Stage 2
- Stage 3

Non-REM (NREM sleep)

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

This is the first stage of NREM sleep. People show slow heart rate and reduced muscle tension during this stage. As drowsiness begins, the time spent in alpha rhythm decreases and the EEG shows waves with much smaller amplitude and irregular frequency. Vertex spikes also appear
- Vertex spike

Stage 1 of non-REM sleep

This is the second stage of NREM sleep. Many people deny the fact that they were asleep when awakened during the first two stages of sleep.
- Sleep spindle
- K complex

Stage 2 of non-REM sleep

NREM3. Defined by presence of large amplitude, very slow delta waves. Often bundled with stage 4 of sleep due to their similarities and called slow-wave sleep. These slow waves represent widespread cortical activity synchronization. 
- Delta wave
- Night terror

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