Personally, I think it's a good idea to have some coffee before the exam, as long as the amount of a cup of coffee is below 130ml.
No more than three cups a day is enough.
I advise you not to drink too much coffee. It's no problem not to drink more than 400ml a day. Drinking too much has a bad effect, that is, side effects.
If you drink instant coffee, that's no problem. You can drink more if you like.
Because instant coffee is not real coffee, but a drink with similar taste.
The landlord hereby declares that it is best to drink freshly ground and brewed coffee.
Instant coffee can't refresh you, it's just a taste.