Back when I was working as a 911 operator, one of the shifts was from 11 PM to 7 AM. I would typically drink at least one pot of coffee on this shift and sometimes, if it was a quiet night, drink a second one. At 7 AM I would go home and jump into bed. I would be asleep within 5-10 minutes. While the caffeine helped (a little) keeping me awake while working, it didn't seem to keep me from sleeping when it was time to sleep.