Washing your hair in the morning is better because you have more time to allow the hair to dry off.
When you wash your hair at night and it's not dry all the way it can lead to knots and and becoming tangled when you sleep.
Although you can prevent that usually by using a hair dryer to make sure your hair is all the way dry before going to bed but I prefer to just wash my hair in the morning when I shower again before work.
Then I don't have to worry about drying my hair out as fast and can let the natural air dry my hair which is much better for it.
But you can wash your at night if you want too or just wait until the morning to wash your hair as other than it becoming knotted and tangled there's no other problem with doing it either way.