I live in South Texas and have noticed a common way that a lot of the older (25 - 35 years) 2 story homes have been set up with AC and duct work. I have one myself, my youngest daughter just purchased one further south in Texas, my next door neighbors house is the same and I'm sure a lot of homes built in this area and in the era mentioned before are similary equiped.

The things in common of the homes are as follows---ALL" bedrooms are upstairs----the thermostat is located upstairs just off the stairwell landing---the condenser is outside of course, in all cases only (1) one unit cooling both floors---the heater/airhandler/evap coils are in the attic---the duct work is the common trunk with branches---the return air is almost always in the stairwell up about half way up to pull air from both floors.

With this common duct system which is 90% of the time square duct board for the trunk and solid galv. pipe wrapped in insulation for the branches. Usually the trunk line runs the length of the attic with tie-ins being made off the trunk with the galv duct line making a short run and then a 90 into each of the upstairs bedrooms and baths. There are also tie-ins off the main trunk which 90 and go straight down through hidden chase areas (master closets--behind the bathroom door, etc., etc) but to make a long story short, they have these ducts that are going down stairs hidden all over the upstairs that in most cases go right through a bedroom closet heading to the ceiling on the first floor right below your sweating feet where no one is needing the air after 10 or 11 at night. When you go to bed at night and have gone upstairs to do so---why can't these ducts that are hidden in the closets be diverted to the upstairs bedrooms with some kind of mechanism that will close off that duct going downstairs only for the night. I know you could call in the army and have the whole thing zoned for a couple 2-3 thousand or put in 2 units , but someone has had to address this simple problem before and if they haven't--they should as this would go a long way to level out the air temperature in a home if not for any other time but sleeping. In south Texas you can pile your attic full of insulation and use all the latest gadgets they have on the market and you're still going to have a difference in temp from 1st to 2nd floor. I could easily cut the duct that is running through my closet, throw a tee into it and install a register in my bedroom to tap into the ac going dowstairs but that would be a manual way of doing it--what automatic way is there to do it that would run off a thermostat set in only your bedroom?