The level of alertness falls, concentration deteriorates, and attention for prolonged monotonous tasks shortens (Table 6.2). The subject is usually distractible and mood changes, particularly irritability, are common. These deficits can be partially offset by an increased effort and by caffeine and other wakefulness promoting drugs.
Stereotyped behaviours with a loss of innovative responses to stimuli, a more limited vocabulary , and a loss of mental creativity and of flexibility of thought processes develop . Working memory is reduced, probably because of active suppression in order to attempt to protect executive functioning. Word fluency is reduced with shorter sentences and preservation and repetition of statements is common. These changes probably involve active inhibition and suppression as well as a failure to generate recall of the appropriate words. There is a reduction in the ability to think abstractly and of verbal creativity. Planning and executive functions are impaired, thoughts are disorganized and there is a perception that it is difficult to think clearly. There may also be disinhibition of mood control, often with a loss of empathy, irritability and a feeling of suspicion. Psychosis is not a feature of sleepiness.
Hallucinations due to altered perceptions of reality or to intrusion of REM sleep into wakefulness may appear and there is a deterioration in short-term memory. Illusions are usually visual, but not auditory. There is also a reduction in the visual field and the ability to search the visual field for important features.
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