As we sleep our minds and bodies pass through several different stages. In fact there are five different stages of sleep, cleverly named stages 1, 2, 3, 4 and REM (Rapid Eye Movement).
Sleep begins as the mind enters into stage 1, which is a very light sleep from which a person can be easily startled and awakened. The heart rate slows, movements slow, and the body temperature drops. You may have noticed on warm evenings that, when the temperature is uncomfortably warm, your body begins to cool even before you actually enter into sleep mode.