Background: Normal young individuals excrete more sodium in their urine during the day than overnight, but the reverse occurs in older individuals with nocturia. The reason is unknown. Methods: First, a self-study was performed, determining the relation between morning and night weight change as an index of volume expansion. This was followed for 1 year and modified, in separate experiments, by assessing either a diuretic (furosemide 40 mg) or recumbency. Weight and in some instances ankle circumference were measured, and day and night urines collected. Second, a community study of 102 individuals was done, measuring circadian weight change and nocturia for 3 days in each subject. Third, measurement of day and night urine electrolytes and weight change was performed in 10 non-nocturics and 12 matched nocturics (age ≧60 years). Results: Salt and water retention occurs during the day and natriuresis and diuresis overnight. Nocturia occurs when weight gain is greater. It is prevented by an afternoon diuretic or daytime recumbency. Conclusions: Idiopathic nocturia is due to daytime volume expansion associated with the upright position. It is hypothesized that this is caused by sodium retention during the day mediated by renal nerve sympathetic activity which together with angiotensin II acts on the kidney to increase tubular sodium reabsorption either directly or by reducing daytime glomerular filtration rate.