feeding from one breast

Rachel
Hi moms.
sometimes my LO only feeds from one breast for about 20 minutes and then falls asleep and does not want to feed off the other. He seems very content but I'm afraid he's not eating enough. Does this happen to anyone else? And what do you do ?? do you wake the baby up or just let them sleep until they wake up on their own for more?   
Usually when I give breastmilk in a bottle he drinks 3 1/4 ounces. So I want to make sure he's eating enough.