ABSTRACT
Sleepiness is a basic physiological need comparable to hunger or thirst, which is satisfied by sleeping, eating, or drinking and thus serves survival of the individual organism. Physiological sleepiness increases while being awake and underlies a circadian rhythm according to the two-process model. The state of sleepiness or drowsiness is a condition between wakefulness and overt sleep. The behavioural indicators are yawning, reduced activity, ptosis, eye rubbing, head drooping and the like. Sleepiness is a complex condition with different causes and consequences, comparable to pain. Whether sleepiness is uni-dimensional, varying only in severity, or multidimensional varying displace separation sign and change word sequence also qualitatively depending for example on REM-or NREM sleep pressure or related on tasks dependent alerting factors, has not yet been clarified (1).
