Yeah, most people, if they reduce/quit caffeine intake by early evening, can get to sleep okay later than night. Most people. I have an uncle who is so sensitive to caffeine that he can't drink any beverage containing it without getting shaky and ill. Biochemically speaking, caffeine binds to specific receptor sites in the brain that are normally receptors for a specific chemical our bodies release after being awake all night. It's a naturally produced chemical which, when it binds with those receptor sites, prompts yawning, weariness, lessening of focus, etc.: all the symptoms we experience and say, "I think it's time to hit the hay..."