Why does my Mom totally saturate her diaper at bedtime?

Thank u all for the input. :)

Guessing here....
She might need to go before bed and can't feel it anymore. Once she's asleep, everything relaxes and empties out. There are overnight inserts you can get through a medical supply business, that will help with overnight heavy - absorption. They look like a waist-bandless version but they don't have a completely soak-proof outer layer because they are meant to be worn inside another adult undergarment.

Mom's doc said the kidneys relax at night and continue to produce urine therefore we should push fluids from the morning and taper off at 5pm-ish to allow her bladder to empty prior to bedtime. It helps.

At least you can be certain she IS getting enough fluid intake! Does she take a diuretic at bedtime, that would then work start to overnight as well?

Francis, drinking water isn't the only way one gets fluids into their system. Canned food, frozen food, and fresh foods will contain some type of liquids.

For myself I have about a 1/2 cup of water or soda in the evening, but by the time morning rolls round, it feels like Niagara Falls.