Stages of Sleep

Stages of Sleep

Introduction to Sleep Stages

• Sleep stages are classified using an electroencephalogram (EEG).
• An EEG measures electrical activity in the brain, displaying voltage fluctuations as waveforms with variable frequency and amplitude.
• The American Academy of Sleep Medicine identifies four stages of sleep.

Stage 1: Light Sleep

• Initial stage of sleep when a person begins to fall asleep.
• EEG recordings show low amplitude waves with mixed, mostly high frequencies.

Stage 2: Light Sleep

• Characterized by specific phenomena observed on an EEG:
  • Sleep spindles: Trains of high-frequency waves.
  • K complexes: Biphasic waves that stand out from the background EEG.

Stage 3: Slow Wave Sleep (Deep Sleep)

• Also known as slow-wave sleep or deep sleep.
• Delta waves, which are low frequency and high amplitude waves, constitute at least 20% of brain activity.
• Considered crucial for overall restfulness.

REM Sleep

• After Stage 3, the sleeper cycles back through Stages 2 and 1 before entering REM sleep.
• EEG activity during REM sleep resembles that of Stage 1 or restful waking.
• Muscles are completely relaxed and limp during REM sleep, but the eyes move rapidly.
• This is when vivid dreams are most likely to occur.

Sleep Cycle Repetition

• After REM sleep, a person might awaken briefly but then cycles through the sleep stages again.
• Most people repeat this cycle four to five times per night.
• Each cycle lasts approximately 90 to 110 minutes.