The frequency of these hallucinations may increase with sleep deprivation. A number of stages of visual hallucinations have been identified. It has been suggested that visual hallucinations during sleep deprivation are centrally caused due to stress on the eyes. This is consistent with the finding that visual acuity is unimpaired during sleep deprivation. It has been suggested that the visual hallucinations during sleep deprivation are actually hypnogogic hallucinations that occur in the microsleeps of attention lapses.
Hypnogogic hallucinations are dream-like thoughts that occur at sleep onset.
Disturbances in thought are another common experience in sleep-deprived individuals. These include such things as loss of memory for recent events that occurred during the period of extended wakefulness, rambling and garrulous speech, lack of coherent of logical structure in thought and speech, and difficulty answering questions. In another instance, the task was not completed at all, and was abandoned. It should be noted here that a confounding variable in such tasks is the participants inability to pay attention to the task long enough to perform. It may not be that memory, per se, is impaired.