Abstract
The daily fluctuations in ocular tension were first noted by Sidler Huguenin 1 in 1899 using tactile tension. The observation was confirmed by Maslenikow 2 in 1904, using applanation tonometry. Since then the topic has been widely studied and the 1961 review of Ourgaud and Etienne contains over 300 references. The terminology has caused confusion both in the literature and in the experimen tal design of studies. Diurnal has been used to mean either a day of 24 hours or the hours of daylight (the antonym of noctur nal) . Halberg 4 introduced the word circadian (circa: about; dies: day) to describe continuously operating biologic oscillations having a frequency of about 24 hours. Rhythmic fluctuations characterize a number of functions in addition to the ocu lar tension. More than 50 rhythmically changing cellular and fluid constituents, functional activity and behavioral elements have been studied in man. 5 Oscillation oc curs at all levels of cellular complexity in biology and may well characterize all living material. The widespread interest in oscillations is evidenced by the numerous recent publica tions concerning this subject, including a large number of reviews, 8 two mono graphs, 6 ' 7 and the proceedings of two sym posia. 8 ' 9 The rhythms described include those involved in the function of unicellular organ isms, in the orientation of insects, the growth of plants, and a variety of functions in ver tebrates. In man, a 24-hour, or circadian, cycle is
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