of or pertaining to a biorhythm having a period of less than 24 hours.

1960–65; ultra- + Latin di(ēs) day + -an

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American Heritage
Medical Dictionary

ultradian ul·tra·di·an (ŭl-trā'dē-ən)
Relating to or exhibiting periodic physiological activity that occurs more than once every 24 hours.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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