Many TVs have a light sensor and will dim the display in a dark room. It saves energy and probably makes the brightness more appropriate to the surroundings. (It doesn't adjust the color temperature as far as I know, just the brightness.)
I'd check your settings. On my Samsung, it's under System -> Eco Solution -> Eco Sensor ("Automatically optimizes TV screen brightness in response to the room lighting.")
I'd check your settings. On my Samsung, it's under System -> Eco Solution -> Eco Sensor ("Automatically optimizes TV screen brightness in response to the room lighting.")