Most families watch the bathroom door. Fewer watch the water glass. But the night trip often starts changing in the evening, well before anyone sees a fall. Your mother may stop drinking after supper because every cup means another walk down the hall in the dark. That is a workaround, and it can be a useful clue.
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Track the glass for a few evenings
Use a plain notepad kept near the kitchen. From supper until bed, write down what she drinks, roughly how much, and the time of her last cup before lights out. In the morning, note whether a glass is still full on the nightstand. A few evenings of notes will show a pattern that one bad night will not.
Walk the trip as she would walk it in the dark
Go from her bed to the toilet and back, with the lights off, and ask whether the walk is shaping what she drinks. Look for the usual reasons she might hold back. The light switch may be out of reach from the bed. The floor between the bed and the door may be hard to see. The cup may sit where she has to stand to reach it. The bathroom may be a longer walk than she remembers. If one of these is true, fix that stop first, and then watch the glass again.
Bring the questions to the physician
How much a person drinks is a clinical question, so route it to her doctor. Bring your notes and ask whether her evening intake is enough for her situation. Ask whether any of her current prescriptions change how much she urinates or how thirsty she gets. Ask which changes should prompt a same-day call, such as dizziness on standing, very dark urine, or new confusion. Ask whether she should have a daily amount to aim for, and whether that changes when she is unwell. Do not set a target yourself from a website or from what worked for someone else.
Write down what you see
On one sheet, keep three columns: the time she drinks, what and how much, and the bathroom trips that night. Add one line for anything she says about the walk, such as that it is not worth getting up. Add a line for dizziness when she stands and for any fall, including a near miss. Take the sheet to the appointment, then keep it on the nightstand for the next one.
If she does fall while you are working through this, start with the first ten minutes after a fall. Then return to the glass. When it stays full, the night trip is usually telling you something before the next fall does. Write it down, check the route, and let the physician decide what her fluid needs are.