The brain responds metabolically to reduced/impaired sleep, time spent awake, and time of day (circadian low). All will separately, and particularly together, cause alertness to vary from very high levels down to extreme sleepiness. This will strongly affect performance capacity and safety. Fatigue is now, for example, a more important factor behind road accidents than alcohol and the interaction between the two potentiates their individual risk values several times over. This is mainly seen in transport work but also occurs in industry and health care. Nights on call (for the MD) are related to significantly increased risk for the patients. The high risk also means that the interest in countermeasures has increased. The best ones appear to be taking a nap or consuming caffeine (blocking adenosine receptors in the hypothalamus which should respond to sleep loss, time awake, etc). Another countermeasure is alertness monitors that warn when sleepiness reaches dangerous levels. A number of European and US projects have been focus on the identification of sensors and on the building of efficient warning systems. The most sensitive variables seem to be eye blink duration or derivatives thereof, or lateral variability of lane position in vehicles. Traditional measures, such as the EEG do detect sleepy driving but the sensitivity and specificity is modest. A final type of countermeasure involves fatigue risk management. It has recently come into focus in air transport but is making headway also in road transport and industry. It essentially involves pre-work evaluation of the schedule using some sort of alertness prediction model, training personnel in fatigue risk management and regular monitoring of fatigue through a routine reporting system. In most cases individuals are aware of the current level of sleepiness. However, once the individual has reached a high level of sleepiness there will be no further signal that the limit has been reached and the persistent individual may simply decide to take the chance and trust his/her ability to fight sleepiness. This lack of insight is probably behind many sleep related accidents and the person that causes this type of accident may refuse to admit to any serious sleepiness and blame a "black-out". A truck driver might blame a non-existent moose for his driving off the road. This would in most countries cause an acquittal since the legislation only recognizes guilt if the defendant was aware of his level of sleepiness. Thus, one needs to change the legislation to refer to illegal levels of sleep loss (<=5h) rather than to perceived sleepiness, analogous to drunken driving sentences being based on blood alcohol levels rather than degree of intoxication.