Preparing for a XC : Bathroom Breaks

writes:

Drink A LOT of water between around 4pm and 5pm - more than I wanted
to. Then sip very small amounts for the next 4 hours. I found I needed
to relieve myself quite a few times for the first 2-3 hours, then less
and less so. Then go to bed. I reliably did not need to get up in the
night. No doubt the physiology is different for night vs day, but I'm
betting the same method would work for flying (although I haven't tried
it).

Renal activity does indeed slow during sleep, so you produce less
urine during the night than you do during the day, all else being
equal. Urine production steps up again as you approach the morning
and your usual time for waking up.

However, the procedure you describe is entirely reasonable and will
help avoid nighttime trips to the toilet. If you drink more water
than you need, your kidneys will usually remove it in 2-3 hours at
most. After that, you will simply be fully hydrated, and you won't
produce an excess of urine unless other factors (like caffeine or
cold) intervene.

A consequence of this is that sleeping in the cockpit would allow you
to go longer without a full bladder, but unfortunately this has other
undesirable effects.

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