Caffeine depletes glycogen quickly. Glycogen is used by the liver to filtre the body's blood. We only produce a certain amount of it in a 24 hour cycle, so if you have used it up filtering high amounts of caffeine, the liver will keep you awake until it has enough glycogen to continue its filtering role. This is most commonly experienced between 3 am and 4 am. So, for anyone else who make read this thread and regularly can't sleep during that time...look to helping the liver out by reducing components that it must process. In my experience, caffeine, alcohol, and "over the counter pain relievers" are especially notorious for this.