After a period of microsleep intervals, subjects often report feeling as if they are functioning at full awareness. However, performance decrements have been reported in color and opposite-naming, body and hand steadiness, mental arithmetic, reading, and visual and auditory reaction time. At this point, researchers begin to observe indication of perceptual disturbances. Borbely suggests that at this point, the line between waking and sleeping is blurred such that the types of hallucinations that occur during sleep invade the waking state. Evidence that the subject experience illusion, incorrect perception of existing objects, and hallucinations, perception of non-existent objects, is observed at this point.
Participants report that the surfaces of objects begin to waver or that cobwebs appear on the floor.
Often times patients will see faces come and go or experience auditory illusions, like that the sound of running water is actually people talking. Subjects may also remark that they are wearing a hat, but are not, due to the sensation of a ring pressure around the forehead, an occurrence sometimes called the hat phenomenon. A fter fourth day of total sleep deprivation, researchers observe delusions in addition to illusion and hallucinations. Participants often show signs of paranoia and suspicion, indicating their belief that events and conversations are going on behind their backs that they are not being told about. They may experience heightened suggestibility, meaning that they are more likely to believe things that they would be less credulous about in a non-sleep deprived state.