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?

  • I edited to make more consistent with current scope, since the answer was pretty good. Still not ideal, but at least better! – DoctorWhom Oct 11 at 7:16
up vote 9 down vote accepted

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 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.

  • If you include arginine vasopressin in your answer, I think that very nicely sums up the "why don't I wake up to urinate" part, but not the "why do I urinate soon again after I get up" part. :) – anongoodnurse May 20 '15 at 17:18
  • That's very interesting - thanks for the answer, and I appreciate the citations! – Bungle May 20 '15 at 18:25
  • @anongoodnurse Well that's one of the hormone systems included in the link in the second bullet point in this table, so if anyone wants to expand on that, there's enough resources, I'd say... – cirko May 20 '15 at 21:22

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