Elderly men sometimes complain that they have difficulty of urination during sleep period or first voiding of the day. The diurnal difference of maximal urine flow rate (Qmax) has been indicated in the elderly men with lower urinary tract symptoms (ref. 1), but little is known whether there is a difference or influence of daylight on Qmax in healthy young men. Here we show the diurnal difference of healthy young men in a controlled stable environment with varying daylight conditions.

This study was conducted simultaneously with the experiments previously described (ref. 2). Twenty-one healthy young men (21−27 years old) were recruited from Kyoto university between November 2019 and February 2020. They participated in a 4-day study involving daytime (08:00-18:00 h) exposure in two light conditions, dim (< 50 lx) and bright (~ 2500 lx), in a random order (4-day study x2). Every urination was assessed by P-Flowdiary ® (Micronix Inc., Kyoto, Japan). Participants were allowed to urinate freely in the 2nd day of the experiment, while in the 3rd day they were directed to urinate at the fixed time of every 3 ~ 4 h. The Qmax at 04:00 h was compared with that at the closest amount of voided volume in the daytime between 11:00 h and 18:00 h (matched Day: mDay) because the Qmax is known to correlate with the voided volume. Identical meals (each 700 kcal, 60% from carbohydrates, 25% from fat, and 15% from protein; 3.8 g salt) were provided to participants at 08:00 h, 13:00 h, and 18:00 h. Participants consumed 25 ml/kg body weight per day of water every 2 h from 08:00 h to 24:00 h. Although they were not expected to follow a strict daily routine, to prevent extremes in activity, participants were not permitted to engage in strenuous physical exercise or to take a nap.

The mean age and BMI of the participants was 23.7 years old and 21.4, respectively. Three participants were excluded because of the missing data of Qmax for the experiment of free urination. The Qmax of after bed-11:00 h was significantly lower than those of 11:00-17:00 h, 17:00-23:00 h and before bed in the dim daylight experiment of free urination (p<0.05 each, Dunnett's multiple comparison test, One-way ANOVA) and of 17:00-23:00 h in the bright daylight condition (p<0.01, Dunnett's multiple comparison test, One-way ANOVA) (Fig. 1). The Qmax of before bed in the dim light daylight condition was significantly higher than that in the bright daylight condition (p<0.05, Two-way repeated measures ANOVA followed by Sidak’s multiple comparison test) (Fig. 1). Voided volume or average flow rate of them were not significantly different. As for fixed time urination, the Qmax at 04:00 h was significantly lower than that at mDay both under the dim daylight condition and the bright daylight condition (p<0.05 and p<0.01, respectively. Student’s t-test) (Fig. 2).

Clear diurnal difference of Qmax existed in healthy young men under dim daylight conditions as well as bright daylight conditions. The bright daylight conditions significantly decreased the Qmax before bed compared with the dim daylight conditions, which may indicate that the bright daylight exposure can induce a phase shift advance in the diurnal variation of Qmax as like Na, Cl, UA excretion rhythm shift detected by Nakamoto et al (ref. 2).  However, the mechanism of action of daylight exposure on Qmax before bed is unknown. It is supposed that this diurnal difference is derived from the involvement of diurnal difference of detrusor contraction, though pressure flow study is required to examine it.

Healthy young men had a clear diurnal difference of Qmax that decreased during sleep period and morning. The bright daylight exposure may induce a phase shift advance in Qmax before bed. Given these results of healthy young individuals, the diurnal difference of the Qmax is considered a physiological phenomenon that is instinctive in humans, i.e., circadian rhythm of our body.

Witjes WP, Wijkstra H, Debruyne FM, de la Rosette JJ Urology 50:221-228 1997
Nakamoto I, Uiji S, Okata R, Endo H, Tohyama S, Nitta R, et al. Diurnal rhythms of urine volume and electrolyte excretion in healthy young men under differing intensities of daytime light exposure. Sci Rep 11:13097 2021

Continence 2S2 (2022) 100246
DOI: 10.1016/j.cont.2022.100246