Biological rhythms exist in a rich variety and almost bewildering profusion to attend and equilibrate the physiology of man. Such dynamic phasic activity appears not only to be intimately concerned with the phenomenology of life and biological processes in general, but is also to be found in purely chemical systems (Hedges and Meyers, 1926). Modalities of the periodicities associated with life can be divided into those external to the organism—including diurnal and climatic variation, sun-spot activity, etc., and into those inherent within the individual such as the respiratory and cardiac rhythms, the menstrual cycle, sleep and awakening. Only less well marked are certain psychological periodicities such as “cyclothymic “variations in mood and personality. In the course of the present century much painstaking research has attempted to link external with internal rhythmic activities, significant correlations being adduced between seasonal variation and, for example, the incidence of psychiatric disorder (Huntington, 1915), immunity from disease (Spencer and Melroy, 1943, Webster, 1944), temperament and behaviour (Petersen, 1934-36; Mills, 1942), and an impressive array of biochemical and physiological variables ranging from blood pH, lactic acid and protein to breath-holding time, plethysmography, tests of hand strength and fatigability, dark adaptation time and various tests of urinary function (Petersen, 1947).