From Sleep Loss and Speech, Yvonne Harrison & James A. Horne, July 1997

After 36 hours without sleep, subjects showed a tendency toward repetition within a semantic category. For example, the letter n might elicit a string of conceptually similar descriptors such as "nil, nought, none, nothing, and no one."

The same letter at the first session was more likely to cue unrelated, concrete nouns such as "netball, nail, nose, nicotine, and newt."

Interestingly, following 36 hours of sleep loss, words were frequently produced in bursts, or chains, separated by lengthy periods of silence.

Sleep loss may encourage more rigid thinking, so once a semantic category has been activated within the memory, subjects have greater difficulty transferring attention to unrelated material.