Historical Perspective In 1842, in a paper entitled "Further Development
of a General Law of Vital Periodicity", which was part of a series of
seven Lancet papers on periodicities in health and disease, Thomas
Laycock wrote: "As everything finite must have a period within which its
existence is circumscribed, so every period so circumscribing the
finite, being a measure of time, must be divisible into lesser periods.
But it has always been found easier to reason from generals to
particulars than to ascend from particulars to generals, especially in
questions in- volving the phenomena of life" (p. 423). From a historical
perspec- tive, Laycock's insight indeed anticipated the progress of
chronobio- logic research. In spite of the abundant evidence pointing at
the existence of short-term rhythms with periodicities much shorter than
24 hours, termed "ultradian" rhythms after Halberg (1964), it has gen-
erally been found much easier to investigate circadian rather than uI-
tradian rhythms. In m st cases, ultradian rhythms have been ignored, or
dismissed as insignificant phenomena, even in cases where they could be
easily "eyeballed" in the data. Laycock himself believed that the most
basic periodicity in vital phenomena was 12 hours. Short-term
fluctuations in the levels of certain behaviors, which could not be
accounted for by external stimulation or by internal stimuli, have been
known to ethologists and other observers of animal behavior for many
years.