It basically dims the screen and keyboard so u can sleep. It also turns off ringers and the led so it dosent bother you when your trying to get some beauty rest.
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Entering bedside mode(depending on the options you have selected) will turn off all sounds, turn off the led and dim the screen for you. While you have the clock up >> hit the bb menu button and select options.From there you can set how you would like bedside mode to behave. In a nutshell it keeps the phone quite while you sleep.