Being a little short on fluid is one of the most common reasons an adult feels flat by mid-afternoon. It shows up as a headache, a dip in focus, or a craving that feels like hunger but isn't. Thirst is a late signal — by the time you notice it, you've been behind for a while.
A reasonable daily target is roughly half your body weight in ounces, more if you sweat, train, drink a lot of coffee, or live somewhere hot or high. On a GLP-1 medication, fluid matters even more: appetite drops, food volume drops, and a lot of the water you used to get from meals disappears with it.
Sip across the day instead of drinking a litre at once. A bottle you refill on a schedule beats trying to remember. Pale yellow urine is the honest check — dark means catch up, completely clear all day usually means you're overdoing it.
Coffee and tea count toward your fluid. Alcohol works against it. Sugary drinks add calories without adding much you actually needed.