There are a number of items that influence the degree to which sleep loss effects behavior. Age is also an important factor in behavior during sleep deprivation. Younger rats could withstand more exercise during sleep deprivation than older rats.
Motivation is also a decisive factor in an individual s subjective perception of sleepiness.
Individuals with high motivation tend to deny sleepiness more than those who have low motivation. They will rate themselves much higher on fatigue than on sleepiness, and their self-ratings of sleepiness will be much lower than the rating of sleepiness by an observer. On certain projective tests, including the Thematic Apperception Test , highly motivated sleep deprived subjects will give fewer sleep themes than control subjects. There seems to be a rough cut-off point of about 40 hours of wakefulness after which the effects of sleep deprivation become more evident. Kleitman argues that after 48-60 hours of sleep deprivation, the term wakefulness loses its meaning.