Final answer:
Sleep is measured using EEG, which shows brain wave activity, identifying four stages of sleep: three NREM stages followed by REM sleep. The sleep cycle, lasting about 90 minutes, repeats throughout the night, with REM periods lengthening towards the morning.
Step-by-step explanation:
How Sleep is Measured and the Stages of Sleep:
Sleep can be measured using electroencephalography (EEG), which visualizes brain wave activity. As we transition from wakefulness to sleep, brain waves change in both frequency and amplitude, allowing us to differentiate between different stages of sleep. There are two general types of sleep: Rapid Eye Movement (REM) sleep and Non-REM (NREM) sleep, each with distinct brain wave patterns and physiological responses.
The Four Stages of Sleep:
People typically progress through four stages of sleep multiple times during a sleep cycle. The first three stages are considered NREM sleep:
- Stage 1: This stage is a transitional phase between wakefulness and sleep, characterized by a slowdown in respiration and heartbeat, with brain wave activity decreasing in frequency and increasing in amplitude.
- Stage 2: Sleep deepens, and the body begins to disengage from its surroundings. Sleep spindles and K-complexes appear in the brain wave patterns.
- Stages 3 and Stage 4: Often referred to as slow-wave sleep (SWS) due to the presence of delta waves. It's a restorative sleep phase, and waking someone up during this stage can be difficult.
Following these stages, REM sleep occurs, which involves rapid eye movements, near-paralysis of the muscles, and vivid dreaming. The brain wave activity during REM sleep is similar to that of being awake. As the night progresses, the duration of each REM period increases, with longer periods occurring in the second half of the night.
Sleep Cycle Progression:
Throughout the night, an individual will cycle through the NREM stages and into REM sleep several times. These cycles last about 90 minutes each. As the night progresses, REM sleep periods become longer, and deep, slow-wave sleep decreases, especially in the second half of the night.