Because it shouldn't, right?
I usually drink a 44 oz "cup" of coffee every day between 8:am and noon. I rarely take any other caffeine. But at certain times of the year I have a lot of trouble with insomnia. A double espresso shot fixes the problem. Espresso about 30 minutes before bedtime and I sleep like a baby from 10-6. No coffee? Fitful, broken sleep for about 4 hours before I just get frustrated and climb out of bed.
I'm an unmedicated bipolar guy, if that matters.