Sleep (modules 8-10: electroencephalogram (eeg) is a device that uses electrodes attached to a person"s scalp to record electrical activity of that person"s brain. Eeg can show activity by displaying it on a graph. Amplitude, which is the height of the wave (measured in volts). Frequency is the number occurrences/sec (measured in hz). Eeg is used to assess stages of sleep: recommendations for sleep (for 18-25 years of age): 7-9 hours/day; and not any less than 6 hours, or more than 11 hours. Stages of sleep (measured using eeg: nrem stage 1 - light sleep that occurs between sleep and wakefulness, and sometimes between periods of deep sleep and rem. Eeg waves transition from alpha waves (common in the awake state) to theta waves. In this stage, hypnic jerks (sudden twitches) and hypnagogic hallucinations can occur. During this stage you lose some muscle tone: nrem stage 2.