Circadian rhythms of urinary excretion: the relationship between the amount excreted and the circadian changes.

1. The circadian rhythms of urinary excretion of water, sodium, chloride, potassium, urate, calcium and phosphate have been studied in several groups of volunteers. 2. These rhythms have been measured: under nychthemeral and constant routine regimens; while subjects were in an Isolation Unit or allowed free egress into society; with spontaneous changes in dietary intake; or after potassium-loading. 3. A direct relationship between 24 h mean rate of excretion and range of excretion was found in all circumstances and for all variables; this relationship was found also when the mesor and amplitude of the cosine curve best describing each 24 h of data were considered. 4. These relationships derive from the observation that, with increases in 24 h mean rates of excretion, nocturnal rates increased less than diurnal rates. 5. This differential sensitivity as between the night and day times has both endogenous and exogenous components. 6. It is suggested that circadian rhythms of urinary excretion result at least partially from this differential sensitivity of the kidney to homeostatic control mechanisms.