All I can recommend for John is plenty of fresh water (a little more often than you think you need it), don’t eat anything fatty or sugary (which comes sadly close to “don’t eat” these days), plenty of relaxation.
By “relaxation” I mean put yourself in a state of mind where you’re not stressing about anything — exactly the recipe you don’t want to face with a massive headache, but it does work.
Other than that, one thing which seems to help most people to relax is to understand that they’ve done relatively well recently, & that they’re gently & widely respected. Again, not something you want to face with so many whingers about — but at least Jon has a head-start in that area.
Oh, yes . . . don’t smack your head into anything. The consequences can be kind of lasting & maybe even terminal — & won’t help your headache no matter how tempting it may seem. (-: I speak from experience, there :-)