Sleep Cycle
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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.
Stage 3 and 4 Sleep
The third and fourth stages combined consist of 30 minutes of deep sleep when your brain produces super slow delta waves. The difference between the two stages is the increased amount of delta waves in Stage 4. During this stage of slow-wave sleep, your pituitary gland releases growth hormones, your muscles relax and your body rejuvenates.
After Stage 4, the cycle repeats itself in reverse order - from Stage 4 to Stage 3 to Stage 2. And after about 90 minutes of sleep, you enter REM sleep.
REM Sleep
When REM, or rapid eye movement happens, your closed eyes move back and forth and you dream. Your breathing and heart rate increase, although on the outside you appear calm and asleep.
After the REM stage, you return to Stage 2 sleep and the cycle continues. You may complete up to six 90-minute sleep cycles in one night. The length of your REM sleep tends to increase from 10-20 minutes as the night progresses. Thus, dreaming tends to get more intense as night approaches morning.
Disorders
Sleep disorders can disrupt your sleep cycle. Many sleep disorders have biological causes, but some have psychological causes. For example, insomnia, or the inability to fall asleep, can be brought on by emotional stress.
Review
You've learned about the five stages of normal sleep, which are linked to your daily circadian rhythm. When you sleep, your brainwaves get longer and slower at first, but short bursts of brain activity called sleep spindles happen in Stage 2 sleep. Your body rejuvenates during the deep sleep of Stages 3 and 4, and you do most of your dreaming during REM sleep. Sweet dreams