Caffeine is one of the few genuinely well-supported performance aids: it improves focus, effort tolerance, and how hard a training session feels. Most healthy adults handle up to about 400 mg a day — roughly three to four cups of coffee — without trouble.
Timing does more damage than dose. Caffeine has a half-life of around five to six hours, so a 4pm coffee still has a quarter of itself in you at bedtime. A cut-off eight hours before bed protects the sleep that's doing most of the work for your body composition.
Coffee on an empty stomach flattens appetite for a couple of hours and then returns it with interest, usually mid-afternoon. If your mornings are coffee-only and your evenings are a raid on the pantry, that's the mechanism. Have caffeine alongside protein, not instead of breakfast.
Watch the stacking: pre-workout, an energy drink, and coffee in the same day adds up fast, and jitters, a racing heart, or anxiety are the dose telling you it's too high. On a GLP-1, caffeine can worsen nausea and reflux on an empty stomach — food first, coffee second. If you're pregnant, on blood-pressure medication, or managing an anxiety disorder, get a personal number from your prescriber rather than using the general one.
One more honest note: caffeine masks under-eating and under-sleeping rather than fixing either. If you need it to function at 10am, the fix is usually breakfast and bedtime, not a bigger cup.