When you fall asleep, your brain and body go through five stages of sleep in one sleep cycle. These stages of sleep have been classified by monitoring muscle tone, eye movements, and electrical activity of the brain using an electroencephalogram (EEG). EEG readings are able to measure brain activating by placing a number of sensors on the scalp.
These sensors measure brain waves and classify them according to speed. Alpha-rhythms have the highest frequency, followed by beta-rhythms. Theta and delta waves are the slowest having the smallest frequency. A typical sleep cycle lasts around 90 minutes or so, during which the brain goes through each type of brain wave.
The sleep cycle continues this process until it has been repeated approximately five or six times during the course of the night.