K.M.
They make a fan-type blind that will cover the arch ($$$), or you could try just a decorative fan (cheap in Chinatown) stuck up there for the time being and take it down when she is no longer a daytime napper.
If you want to time her waking more precisely, darken the room completely and then get one of those natural light alarm clocks that cycles from dim to bright light over 30 min (fully bright at target wakeup time.)