overiding a thermostat

I have an 18000BTU Fedders room airconditioner. It has a digital thermostat setting. Usually every night I set the temp on 68 degrees to sleep by but invariably the temp just automaticaly goes up to 88 degrees and then it is just sitting there blowing hot air. Is there any way I can overide everything and just cut the air on and it just blow cold air instead of any kind of thermostat setting? Please help!!! I don't want anything but 68 degree air as I only run this thing at night.