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About 20% of human subjects, living under constant conditions (without time cues) showed rhythms of activity and temperature with greatly different periods ("internal desynchronization"). Psychological testing indicated that these physiologically internally desynchronized subjects had a significantly greater tendency for neuroticism and showed a significantly greater tendency to complain of physical ailments than the other 80% of the subjects, which were not internally desynchronized with regard to rhythm of activity and temperature.
R. Lund (Wed,) studied this question.