In the following sections, look at the brain activity for each sleep stage, and then read their interpretations to see how different your brain activity is during each stage of sleep.
Brain activity is measured by EEG – electroencephalography – which detects electrical wave patterns in brain cells. Those patterns are defined by how high (i.e., amplitude) and how fast (i.e., frequency) the waves are. High and slow waves = less brain activity. Low and fast waves = more brain activity.
Awake: beta and alpha waves – fast, low amplitude waves (remember: fast and low = more brain activity).
Stage N1 (transition to sleep): Lasts ~5 minutes. Lightest NREM sleep. Eyes move slowly. Muscle activity slows. Easily awakened. Theta brain waves – slower and higher amplitude waves than wakefulness.
Stage N2 (light sleep): Lasts 10-25 minutes. Actual sleep. Eye movement stops, heart rate slows, body temperature decreases. Harder to awaken. Theta wave activity with sudden bursts of brain activity – sleep spindles (sudden, intense increases in wave frequency) and k complexes (sudden, increases in wave amplitude).
Stage N3 (deep sleep): Deepest NREM sleep. Blood is directed away from the brain to the muscles. Difficult to awaken. Brain is less responsive to stimuli. Delta wave activity – slowest and highest amplitude waves (remember: slow and high = less brain activity).
REM sleep: Paradoxical or dream sleep. Eyes move rapidly, breathing slows, heart rate and blood pressure increase but most muscles are paralyzed to prevent acting out dreams. Theta and beta waves.
All of these sleep stages occur in a particular order. The order is normally:
Stage 1 → Stage 2 → Stage 3 → Stage 4 → REM.
This represents one full sleep cycle. A typical 8-hour sleep consists of about 4-5 repeating cycles. There is more deep N3 sleep earlier in the night; the amount of REM sleep increases in the two cycles just before awakening.
