Periodical measurement of urine volume in the bladder during sleep: Temporary volume reduction suggestive of absorption

Objectives To clarify the variance of urine volume in the urinary bladder during sleep. Methods Overnight measurements of urinary bladder capacity during sleep were carried out using transabdominal real‐time 3‐D ultrasound or an automatic capacity recorder, Yuririn, in 24 volunteers of various ages. Results Two types of patterns were observed in the increasing curve of capacity; namely, gradual elevation pattern when overnight urine production was less than functional bladder capacity and steep‐flat pattern when it was more than functional bladder capacity. During steep‐flat pattern, bladder capacity reached levels of the functional bladder capacity quickly, then maintained steadily at that level until morning without awakening. Temporary volume reduction, which might be suggestive of water absorption from urine in the bladder, occurred occasionally, when the volumes of urine reached functional bladder capacity. Conclusions A considerable amount of urine in the bladder diminishes without micturition during sleep to maintain a good level of sleep. This is the first report suggesting the possibility of water absorption from urine within the human urinary bladder in a fully physiological situation.