Lajixetk du milieu intlrieur was firmly entrenched as a physiological concept by Claude Bernard, though Cannon moderated it by entitling it homeostasis. Today we are urged (87) to envisage rather ‘a milieu constantly changing, with a predominantly circadian rhythm, but with many components of longer or shorter duration; any observation may be suspect without a statement of the time of day at which it was made.

The Society for Biological Rhythm, as it is now called, has held a series of conferences. 1 Detailed reference is made below to certain individual contributions to these and other conferences (69, 77, 248, 249). These reports assemble much material and pointed discussion not easily available elsewhere; they survey a wide field of biological rhythms of different periodicities in various organisms, plant and animal. Kleitman’s (I 29) review in 1949 also covered this wide field, although he devoted only 3 of 27 pages to human circadian rhythms, to which this review is largely confined. Work on species other than man are discussed, however, where results have not been, and could not readily be, obtained on man and where matters of a fundamental nature are involved. Of other important reviews since Kleitman’s, one by Aschoff (II) covers a similar field and is full of stimulating ideas, and two others (I 3, IOO) confine themselves to circadian rhythms. Books have been published on biological cycles in