Electroencephalography, and its record, the electroencephalogram (both abbreviated EEG) document the electrical activity of your brain, also known as electromagnetic oscillations. EEGs are one major component of polysomnography, a night-long sleep study usually conducted in a special sleep lab or center. EEGs are one of the best ways of determining what stage of sleep a person is in because the electrical activity of the brain changes depending on the stage of the sleep cycle a person is in. In fact, sleep stages have been defined in large part by EEG recordings. The main exception is REM sleep, in which the characteristic rapid eye movements and lack of muscle tone are detectable without EEG records.
There are four main types of brain waves documented by EEGs, all named for letters of the Greek alphabet: Alpha, Beta, Delta, Theta, and some sources include another called Gamma, or extra high Beta. Each type of brainwave is characteristic of a type of brain activity, and is defined by frequency. Frequency is a term from physics, defined as the number of repetitions of a wave cycle in one second expressed in units called Hertz (named for Heinrich Rudolf Hertz, radio pioneer, and abbreviated Hz) and amplitude. Amplitude refers to the height of the wave above and below a zero line, which represents it power. If you think of it in terms of radio signals, the frequency is the number on the dial or LCD, while the amplitude is how strongly the signal is received.
Beta is highest frequency, but lowest amplitude, Alpha is slightly lower frequency and higher amplitude, Theta, still lower frequency and higher amplitude, and Delta is lowest frequency and highest amplitude. There is a disagreement among sources as to the exact dividing line of frequency between each type of wave. Delta begins just above 0 (anywhere between 0.5 and 1.5 Hz) and extends to 3 - 4 Hz, Theta begins at 3 - 4 and ends at 7 - 8 Hz, Alpha begins at 7 - 8 and ends at 12 - 14 Hz, and Beta begins at 12 - 14 and extends as high as brain waves can be measured.
During the day, when you are active, especially processing information, you are mostly in the Beta state, when you rest after completing an activity, you descend into Alpha, when you are doing something repetitive such as long distance driving, you can be in low Alpha or Theta, and when you are deeply asleep, you are in Delta. However, when you begin to dream actively, as in REM sleep, you move from Delta to Alpha. Waking up from sleep, you ascend back from Delta through Theta and Alpha eventually to Beta, just the opposite of going to sleep, when you descend from Beta or Alpha to Theta and then Delta.
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