The first stage of sleep has a brainwave frequency of 4 - 8 Hz and is the transitional state between being awake and falling asleep. It consists mainly of high amplitude, but low frequency, theta waves. There are also short periods of alpha waves similar to those experienced when awake and this stage usually only lasts for a few minutes.

In stage two (8 - 15 hz), we experience sleep spindles which are peaks of the brainwaves becoming higher. Spindles are followed by k-complexes which peak suddenly before quickly descening then picking back up. This stage also usually only lasts for a few minutes.

The third stage of sleep (2 - 4 Hz) is delta or deep sleep and over 50% of our brainwaves are delta waves and are at a lower frequency than theta waves. 20 - 50% of these brainwaves are delta waves and the rest are theta waves.

The fourth stage of sleep (0.5 - 2 Hz) is also known as delta or deep sleep and more than 50% of brainwaves are delta waves, the rest are theta. This is the deepest form of sleep before REM (Rapid Eye Movement) occurs, which occurs after these stages have been reversed.

The final stage of sleep is REM (12 Hz +) and occurs when beta waves are of a high frequency and the brain becomes quite active in both sleep and a waking state. This stage may come with muscular twitches, the heart may beat faster and breathing may become much deeper and rapid. Dreams are much more realistic when in this state.

Anyone interested in the human mind is advised to read some of my other hubs.