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1.
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.
by roast beanz June 24, 2011
 
2.
2. A window of opportunity from early a.m. until usually about noon or 1 p.m., in which drinking caffeine is ok. For the recovering caffeine addict who can no longer drink caffeine after mid-morning or early afternoon, lest he be force into some sort of crazy all-nighter bullshit, in which he's either up all night unable to sleep or simply brewing pot after pot in order to generate an next morning mania for work.
Dude 1: I can't have coffee at night anymore.
Dude 2: Oh? Why's that?
Dude 1: Waaaay too late. I'm outside today's caffeine window.
Dude 2: Ha! Whatever...

Dude 1 goes and gets a coffee anyway.
by java bo jangles July 03, 2011