What is the Sleep Cycle? - Definition, Stages & Disorders

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  • 1:11 Stage 1 Sleep
  • 1:45 Stage 2 Sleep
  • 2:06 Stages 3 and 4 Sleep
  • 2:38 REM Sleep
  • 3:07 Disorders
  • 3:22 Lesson Summary
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Lesson Transcript
Instructor: Paul Bautista
What happens when you sleep? Why are dreams typically more intense as morning approaches? This lesson will guide you through the stages of the sleep cycle that make up a full night's rest.

You'll spend about one-third of your life sleeping. Added up, that's more than 9,000 days of sleep! The average time spent sleeping changes over your lifetime. As a newborn, you might have slept 18 hours a day, whereas as an adult, you may sleep about eight hours a night. As you get older, you may only sleep for six hours at a stretch.

But what is sleep, and what goes on in your body when you're asleep? Your sleep-wake cycle is a biological rhythm, or a regularly recurring pattern. More specifically, it's a 24-hour cycle or circadian rhythm that's linked to daylight and darkness. Your circadian clock sets your internal functions like changes in blood pressure, body temperature and metabolism to the local time in predictable ways. For example, you may feel more and more awake as the morning progresses, but then feel less alert as the afternoon wears on, then continue to slow down in the evening as you prepare for sleep. If you've ever traveled to a different time zone, you know the effects that jet lag can have on your biological clock.

When you sleep, your body goes through five stages.

Stage 1 Sleep

During the first 5-minute stage, you transition from wakefulness to sleep. If you had electrodes on your forehead hooked up to an EEG (electroencephalogram), this machine could measure electrical activity in your brain. As you fall asleep, an EEG would show that your electrical brainwaves change from rapid beta waves when you are awake to slower alpha waves as you become drowsy, then finally to even longer theta waves as you drift off to sleep. During Stage 1 sleep, you might twitch or feel like you're falling as you begin to drop off to sleep.

Stage 2 Sleep

The next 20-minute stage is a deeper sleep. Your heart rate and breathing slow; your body temperature drops. Whereas, Stage 1 may feel like dozing off, if you were awakened from Stage 2 sleep, you'd actually feel like you'd been asleep. Stage 2 is characterized by sleep spindles, or short bursts of brain activity.

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