Sleeping all starts when a molecular level with a chemical that works with your nerve cells in the brain to help tell the brain when your body is ready to sleep and if you should even still be awake. When you start getting really tired it means sleep is kicking in and removing the toxins in your brain that help you stay awake. There are also stages of sleep, light sleep, deep sleep, REM, and awake. A human experiences all four stages each night.