Why the LCD screen is brightened automatically?

hello. have mbp 2010 17". @ home, connect external monitor. past few days, left machine on backup data. before went bed, pressed f1 several times dim lcd screen complete dark. then, went bed. few minutes later, lcd screen brightened (really scary). has happened several times already. knows happened , how fix this?

 

disable auto brightness in sys prefs.

 

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