YA - man time travels between two time periods whenever he goes to sleep after hitting his head in the shower, other time period is ancient/ medieval times

[Trenton]

one time period is modern times, the other one is ancient with only half the earth being habitable, there's a river that goes all the way around the world that splits it in half, there's a diseased desert horde that attacks and tries taking over regular people. there's a pond that he swims in that cures the disease, he helps these people invent gunpowder. he falls in love with someone and had kids with her. i think the ancient time period ended up being like 1000 years in the future after a nuclear world war. the uninhabitable part of the earth has creatures that can fly and whenever anyone tries crossing the only bridge they get attacked but he ends up over there and tames them i think. when he falls asleep in one time he wakes up in the other but the amount of time spent in each is different