Fatigue is associated with physiological changes in brain wave activity, eye movement, head movement, muscle tone and heart rate. With the onset of fatigue, body temperature, heart rate, blood pressure, respiration rate and adrenalin production are lowered. When fatigued, a person may experience micro-sleeps. Micro-sleeps are brief naps that last for approximately four to five seconds.
One of the most valid indexes of alertness in the driver is the electroencephalography (EEG) . The EEG (electroencephalograph) measures brainwaves of different frequencies within the brain. The electrical activity of the brain is classified according to rhythms. These rhythms are defined in terms of frequency bands including delta (0.5-4 Hz), theta (4-7Hz), alpha (8-13Hz) and beta (13-30Hz). Deltawaves are present during transition to drowsiness and during sleep. Theta rhythms replace the alpha components at the onset of sleep. Beta waves are associated with increased alertness, arousal and excitement .