Forget sconces get the electrical lower like any other box for lamps and no matter what size bed the end tables will hide the wires.

If you install receptacles on either side of the headboard area at normal height (about 12" off the floor), many sconce lights are designed to have the wiring come down the wall and plug into a standard receptacle. They usually have a matching tube or other trim that hides the wires.

I would also consider installing a double box for the receptacles on either side of the bed so that you have up to four plug-ins. Many folks have an alarm clock and cell phone charger on the nightstand, and still need to plug in lights and other bedside items. Or you could use a double box where one half has standard receptacles, and the other half has an insert with USB charge ports.

Bruce