People have reported not being happy with scheduling an Uber ahead of time. I'm trying to remember why. Maybe Uber isn't actually reserving a car and driver for you; you are just telling them the night before that at xxx O'clock they should notify drivers that you need a ride - as opposed to you notifying drivers yourself at xxx O'Clock that you need a ride. The other thing is if they do it too early and you aren't on the curb waiting when the Uber arrives, they can cancel on you and charge you a cancellation fee.
If I'm wrong I assume others will say so.