Sleep stages

Electroencephalographic (EEG) Recordings:

Not unitary process but progression of cycles.
Alpha rhythm	Relaxation, eyes closed
Stage I	During this period we are just falling asleep. Our heart rate begins to slow and our muscles relax. The EEG is irregular and lacks consistency of alpha waves that occur when we are awake and relaxed. It has low voltage with fast activity. REMs are absent, but slow rolling eye movements appear.
Stage II	A deeper sleep than stage one. The EEG show bursts of activity (12-14 Hz sinusoidal waves) called "Spindles", and an occasional sharp rise and fall in amplitude (K-complex). Eye movements are rare.
Stage III	Sleep becomes deeper and spindles disappear from the EEG. The spindles are replaced by long delta waves, heart rate slows and temperature drops. The sleeper is more difficult to wake during this stage, but can be aroused by calling out a familiar name. Often times a loud sound, such as a door slamming, will be ignored. EEG has high amplitude (>75 mV), slow (0.5-2 Hz) waves.
Stage IV	A quantitative increase in delta waves, higher amplitude and slower waves. Delta sleep: Most of time during first half of night Sleep walk
Paradoxical sleep	REM (Rapid Eye Movement) Very deep but like phase I Low voltage Theta waves REM - correlated with dreaming (memory). Starts approximately every 90 minute. Muscle tone disappears Eye movements Adults don't go directly into REM sleep