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Your brain is a surprisingly busy place when you're asleep, and not only while you're dreaming. As it happens, you are moving through the stages of sleep, each of them unique and essential to your well-being.
During stage one, your sleep is light. You may move in and out of sleep, and it doesn't take much to wake you up. In fact if you are wakened, you might swear that you weren't really asleep and you might mean it.
This period usually only lasts for about ten minutes. Brain waves are very slow (theta waves). Your eye movements are slow, and so is your muscle movement. This stage is the frustrating part where you may have leg twitches (also known as myoclonic jerks), or the feeling of falling (hypnagogic hallucination).
During stage two, your eyes stop moving. Your brain waves slow down (alpha waves), though there may be a few intermittent bursts of rapid activity called sleep spindles. Your body temperature decreases and your heart rate slows down. Stage two lasts about twenty minutes.
Stage three is a mixture of delta waves (very slow) and beta waves (smaller and faster). Stage four is characterized by delta waves and lasts about half an hour.
These last two stages are delta sleep (deep sleep). This is when you can sleep through just about anything. Your eyes and muscles are still. This is the domain of bedwetting, night terrors and walking in your sleep. These two stages are often now considered to be one stage.
Most of your dreaming sleep is REM sleep, characterized by shallow, irregular breathing and darting eye movements. REM is also called paradoxical sleep because while your arms and legs are still, as though paralyzed, your brain waves resemble those experienced in a waking state.
Your blood pressure goes up and your heart speeds up. Men have erections.
And this is the time of dreams. If you wake up from REM sleep, this is the time when you will be able to remember details of your dreams.
You won't go from the lightest sleep progressively to the deepest sleep. From stage four, you'll repeat stages three and two before you hit REM.
After a period of time in REM sleep, you'll bounce back to stage two.