Scientists divide sleep into two major types: REM (rapid eye movement) sleep or dreaming sleep, and non-REM or quiet sleep. Surprisingly, they are as different from each other as each one is from waking— yet both may be important for energy.

Non-REM sleep involves three stages. Sleep specialists believe that the last of them—known as deep sleep or slow-wave sleep—is the main time when your body renews and repairs itself. This stage of sleep appears to be the one that plays the greatest role in energy, enhancing your ability to make ATP, the body’s energy molecule. In deep sleep, blood flow is directed less toward your brain, which cools measurably. At the beginning of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissue growth and muscle repair. Researchers have also detected increased blood levels of substances that activate your immune system, raising the possibility that deep sleep helps prepare the body to defend itself against infection.

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