What is the physiological process that would cause one to void more than once in the morning?

Question

I have observed that drinking a full glass of water prior to sleeping leads to urination in the morning, as would be expected.

However, I have also observed that some people urinate more than once in the morning, often just an hour after the first full void.

I theorize that:

• The individual drank more than the bladder can hold, thus the body held onto the excess water until there was more room in the bladder before producing more urine.
• The body retains water overnight for some reason, and begins to move it to the bladder after one wakes up.

What is the physiology of urination that yields this pattern?

Answer 1

In healthy patients, a circadianic rhythm of urine excretion has been observed. In average, it depends on age, sex and total (consumed) fluid volume. Several factors contribute to this rhythm:

• The rhythm of glomerular filtration rate of the kidneys. They decrease filtration of urine at night.
• Many of the hormones which influence renal urine production - and also the parasympathetic/sympathetic nervous system, which controls bladder function - follow observable day and night rhythms.
• The former study also states that there is a possibility for regulatory clock genes, which might directly influence urine production or the urge to urinate. They could be located in the bladder itself or the neurons controlling its function, or in the kidneys.

The role of this rhythm is still unclear. In mice, the circadianic rhythm of urine excretion was lost 2 days into exposure to total darkness.

So, altogether, there is evidence of multifactorial contributions to an observable circadianic rhythm of urine excretion of unknown purpose. But it is clear that at night, your kidneys filtrate and thus produce less, and the tonus of the bladder muscles is also decreased, in summary leading to your bladder preserving more urine for the morning, possibly "to give you some rest" at night. And it is also clear that if you're a man, it is not going to get better with age, and this is very likely due to the increase of their prostate volume.