Udo, Marc, Eric - thanks for your inputs. I looked up "symptoms of
dehydration" with Google and found this:

Symptoms
* Dry or sticky mouth
* Low or no urine output; concentrated urine appears dark yellow
* Not producing tears
* Sunken eyes
Signs and tests (from a physical exam)
* Low blood pressure
* Blood pressure that drops when you go from lying down to standing
* Rapid heart rate
* Poor skin turgor (pinched skin doesn't spring right back into
position)

Udo, I can sleep for 8 hours without a bathroom visit, but my father
couldn't starting at about 70. Marc, you say "noticably dehydrated" -
what symptoms did you exhibit? Eric, my last bathroom visit before
flight usually produces a goodly amount of pale urine and I don't have
a dry mouth, so I don't think I'm dehydrated before flight. I'll keep
alert for signs, though.

-John