Our sleep progresses through four distinct cycles, and important upkeep tasks are going on during each one. We start out in light sleep, also called stage 1, when we can be easily awakened. This is followed by stage 2, when it would be slightly more difficult to wake us, and we are transitioning to deep sleep. During stage 3, deep sleep, most of the important cell repair and system restoration is happening. The fourth stage is rapid-eye movement (REM) sleep, when our brains are very active and dreaming takes place. We move through these cycles 4 or 5 times during a full night's sleep.