One the bedroom door has to stay open unless you have a return in your bedroom..
Second, like others have said, check the insolation above your bedroom. If that's not an issue you may need to spend some money and re duck you home system and move the thermostat near your master bedroom.
Or an easier solution would be to install a split AC system just for your bedroom. Then you could keep your bedroom at 68 at night while the rest of the house is at 78.
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