First Sleep Cycle
Stage	EEG Activity	Conscious Awareness	Level of Physical Arousal	Time in Stage
1	Alpha and theta	Easily aroused	Relaxed, drowsy	A few minutes
	waves
2	Mostly theta waves; Sleep spindles, K complex, beginnings of delta waves	Slight sounds will not arouse	Very relaxed	15 to 20 minutes
3	Delta waves greater than 20% of total brain activity	Difficult to arouse	Heart rate, blood pressure, and breathing are slowed	5 to 10 minutes
4	Delta waves greater than 50% of total brain activity	Very difficult to arouse; oblivious to the world	Heart rate, blood pressure, and breathing are slowed and breathing are at lowest levels	20 to 40 minutes
REM	Brain waves small and fast, similar to wakefulness; rapid eye movements	Similar to stage 2	Elevated physio-logical level; voluntary muscle activity is suppressed	5 to 15 minutes
Combined, the four NREM stages take 50 to 70 minutes. Stages 3 and 4 (slow-wave sleep) occur only during the first two cycles. Each of the 5 to 6 cycles per night lasts about 90 minutes. REM stage becomes progressively longer throughout the night.