A 14-18 Hz wave that appears in bursts during stage 2 sleep.

Sleep spindle

A K-complex is a very high-amplitude pattern of brain activity observed on an EEG during Stage 2 of NREM sleep, visually distinguished by its significantly high amplitude and longer wavelength. It can occur as a reaction to environmental stimuli and is thought to serve as a bridge to higher levels of arousal. K-complexes are also involved in sleep maintenance and memory consolidation.

K-complex

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

University of Rochester

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

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

Stage 3 of non-REM sleep

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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