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.
For example: "Darn, I missed my caffeine window. Time to go ape shit on everyone."
Remember addicts, always have your caffeine handy because the next best thing is crack.