During sleep, our bodies proceed through a series of stages, with each stage serving a function in restoration and rejuvenation of different areas of the body. As people move from waking state through deeper stages of sleep, brainwaves become slower and more rhythmic and we move from awake to non-REM sleep to REM sleep.
After a period of REM sleep, we descend again through the initial stages with a complete cycle of REM and non-REM occurring approximately every 90 minutes. As the night progresses we spend less time in deeper sleep and more time in REM sleep.