There are a number of items that influence the degree to which sleep loss effects behavior.  These include circadian rhythms, situational characteristics, individual traits, and task characteristics (Bonnet, 1993, p. 177).  Age is also an important factor in behavior during sleep deprivation.  Younger rats could withstand more exercise during sleep deprivation than older rats.  These rats also showed greater ability to withstand the anoxia and hypothermia that occurs during sleep deprivation (cited in Kleitman, 1963, p. 219).

    Motivation is also a decisive factor in an individual’s subjective perception of sleepiness.  People who are highly motivated to stay awake have a markedly different attitude towards sleepiness (Morris, Williams, & Lubin, 1960, p. 248).  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.
In a young man who stayed awake 231 hours (interspersed with 5.25 hours of accidental sleep), lack of any significant performance or physiological deficits were attributed to the personality, character, and motivation.

    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.  In some ways a person deprived of this much sleep is awake, but in other ways, their behavior is not like that of true wakefulness.

    In one experiment, after about 40 hours of sleep deprivation all participants shoed some psychotic symptoms, though the severity of these symptoms varied considerably across participants.

    After about 40 hours of sleep deprivation, the will of a person to stay awake lapses in its effectiveness.  An individual must be continually monitored to ensure that sleep does not occur.  Within a nychtemeron (which is one 24-hour period), it is most difficult to remain awake between 4:00 and 6:00 a.m., while it is easier to stay awake in the afternoon and evening.

    One nearly universal aspect of sleep deprivation in humans and animals is that it becomes more difficult to keep an organism awake as the period of sleep deprivation lengthens.  Puppies become progressively more difficult to keep awake the longer they are sleep deprived.  It becomes harder to keep puppies awake in periods of rest, while it is somewhat easier in periods of physical inactivity, or when the attention of the dog was captivated.  However, after 3 or 4 days of deprivation, puppies showed a loss of interest in the environment, and made continual attempts to sleep.

    Human subjects are capable of lying awake in bed during the first night of sleep deprivation.  During and after the second night, it becomes virtually impossible to keep a sleep-deprived person awake if he or she is in bed.  Some form of motor/muscular activity is required to remain awake.  Because of this, one of the challenges that experimenters researching sleep deprivation initially faced was to be able to separate any observable effects as owing to sleep deprivation or the activity that was necessary to produce the sleep deprivation.

    Sleep deprivation may exacerbate any pre-existing condition or tendency.  One case study of a man who had a history of nervous disorders, but who was stable going into the sleep deprivation period.  After two voluntary bouts with sleep deprivation (or 89 and 168 hours), the man was in some ways rendered dysfunctional.  Though his behavior and symptoms during the deprivation period were not drastically different than the behavior of sleep deprived subjects without a history of nervous disorder (including memory lapses, visual and auditory hallucinations, delusions, confusion, and disorganization), the experimenters noted the severity of these symptoms and, to some degree, the persistence of these and other behaviors (including bizarre thoughts, degradation of personal and financial affairs, tardiness, insomnia, attention lapses, irritability, restlessness, and consumption of alcohol) after the period of sleep deprivation had ended.

For overview of Sleep deprivation:

   Effects of Sleep Deprivation
  on Mood
  on Behavior
  on Cognition
  on Physical aspects
  Recovery after sleep deprivation
    Slow Wave Sleep (SWS)
    REM Sleep
    SWS vs. REM