Dozing
Once we doze off, we experience 90-minutes cycles of Non-REM and REM (Rapid Eye Movement) sleep. Non-REM or Delta sleep dominates the first part of a night's sleep, while REM or dreaming sleep commands the second half. The amount of Delta sleep tends to decrease with age, with children experiencing the most Delta sleep and older people experiencing little or none.

Dreams
Using the EEG (electroencephalogram), which measures brain waves, researchers have shown that most people remember their dreams most often when they awake from REM (Rapide Eye Movement). In REM sleep, our eyes flutter inside their lids as if scanning the dreams we're experiencing. What exactly dreams are, and why almost every mammal has them, is still a mystery.

Paralysis
During REM (Rapid Eye Movement) sleep, the brain actively paralyses the body, sending signals to muscles in the limbs, face, and mouth that prevent us from acting out our dreams. When we talk or walk in our sleep, it's often because the brain, for reasons that are not clear, is failing in its attempt to send those paralyzing signals.

Recharging
While we sleep, breathing and heart rate increase and decrease with predictable regularity and chemical and electrical activities in the brain can be higher even than during relaxed wakefulness. But how much sleep do we need? Though 8 hours is not the ideal for everyone, experts say there's one simple way to tell: If you feel refreshed when you wake up, then you've goten enough.