can humans get fleas in their hair
Oh, yeah. Fleas tend to cluster close to the floor, so you're most likely to be bitten about the ankles up to the shin, but if you sleep somewhere where flea infested animals have been, you certainly can get them in your hair.
Yes, humans can get fleas in their hair. However, hair is typically too oily for fleas to survive, so they will not likely live in a person's hair.
Yes, due to lack of proper hygiene it is very much possible for humans to have fleas on their hair. Learn how to get rid of this by visiting http://www.livestrong.com/article/22487-treat-fleas-human-hair/.