Does closing window shades save fuel?
It isn't due to "lowering the demand on the engine bleed" because the air conditioning is done as part of the standard pressurization of the cabin - the air is bled from the engine, cooled to a very low temperature and then heated to the desired temperature for the cabin, so shutting the blinds wouldn't affect that because the air is still cooled and then reheated.  

It is purely for jet-lag and sleeping issues - a single blind open in a darkened cabin can disrupt even those people wearing the eye masks (those things arent perfect) and can disrupt people who have woken to go to the toilet during the cabins "sleep period" (think how horrible it is if you get up in the middle of the night and turn on a bright light - its easier to get back to sleep if you keep lighting muted throughout your activity than it is otherwise).