During your sleep, you go through a number of 90-120 minute cycles. In the first part of each cycle, often referred to as the “deep sleep” phase, your muscles are repaired and your body releases hormones to regulate growth and appetite. This phase is essential for maintaining a healthy body and takes up a larger proportion of your sleep early in the night.
In the second part of each cycle, the REM phase, you dream and your memory is consolidated. This phase is important when you are trying to learn complex tasks. The REM phases start short but get longer as the night progresses.
During a healthy sleeping cycle, people will wake up several times. Normally, these waking moments last for 1 or 2 minutes and are not remembered the next day. However, stress, alcohol and caffeine can cause you to fully wake. If that happens, it often takes a full sleep cycle (90-120 minutes) to fall asleep again.