BACKGROUND: Aging is associated with an increase in sleep complaints, and many older people have difficulty remaining asleep towards the end of the night. One factor causing sleep disruption is waking to urinate, but whether the need to urinate causes awakening or whether walking influences the decision to urinate is unknown. To explore whether there are changes in urine output during the night with aging, we examined data from subjects who participated in circadian rhythm studies in our laboratory
METHODS: Data were obtained from healthy young (25.7±5.2, n=26, 8 women) and older (69.4±4.5; n=26, 12 women) subjects whose study included at least 2 baseline days followed by a constant routine (CR). The CR began at wake time and consisted of 40h of wakeful bed rest, and hourly snacks containing the same amount of fluid. All voids were collected and urine output mL/min) was calculated across the interval between each void. Data were binned hourly and averaged per hour across the 2 groups. We also compared total urine output from the first 16h of the CR to the following 8h.
RESULTS: Both age groups showed a circadian variation in urine output (p<0.01) with lowest output near the beginning of the usual sleep time. There was a difference between the age groups (p<0.01), and an interaction between age and time (p<0.01). The older subjects produced a greater percentage of their daily urine output during the nighttime hours than did the young subjects(p<0.01).
CONCLUSIONS: We observed a significant difference in the circadian pattern of urine output between healthy young and older subjects, even though both groups received a fixed fluid intake each hour and were restricted to wakeful bed rest. Older subjects produced a greater percentage of their daily urine output during the nighttime hours suggesting that age-related differences in the circadin pattern of urine output may contribute to the complaint of sleep disruption and nocturia among older individuals