Sorry, but there is no way to do that. Your BMR calories are always counted. And even if you could ignore calories when sleeping, you would still be getting BMR calories while you are awake.
From How does my Fitbit device calculate my daily activity?
Has this been updated yet? It’s an Issue thousands of people online seem to have just like me! I don’t care about how many calories I burned in the night. I want a fresh start when I wake up. I currently do the math every day from the sleep calories I’ve burned by subtracting them.