The daily time slot in which you must have some form of caffeine otherwise you will get a headache. No amount of caffeine after this window will cure the headache. A common ailment of coffee addicts who need their morning fix before they can function properly.
Dude1: Fuck, I've missed my caffeine window, I feel a headache developing.
Dude2: Rookie error dude, that's you should always keep a few No-Doze handy when you don't have time to grab some coffee.