The quality of your sleep is determined by a process called the sleep-wake cycle.

There are two important parts of the sleep-wake cycle:

1. Slow wave sleep (also known as deep sleep)
2. REM sleep (REM stands for Rapid Eye Movement)

During slow wave sleep the body relaxes, breathing becomes more regular, blood pressure falls, and the brain becomes less responsive to external stimuli, which makes it more difficult to wake up. This phase is critical for renewal and repair of the body. During slow wave sleep, the pituitary gland releases growth hormone, which stimulates tissue growth and muscle repair. Researchers also believe that the body's immune system is repaired during this stage. Slow wave sleep is particularly critical if you're an athlete. You'll often hear about professional athletes like Roger Federer or LeBron James sleeping 11 or 12 hours per night. 8