You can take it off, but it will just estimate the calories you burn then. I would advocate wearing it for a week or so at night so it gets an idea what you are actually going to burn at night, then after that it will just estimate based on the numbers it has.

I wear mine on my leg at night so that I get an idea how well I'm sleeping, but it's about the same burn every night, regardless, unless I'm really restless.

I have both a fit bit(that's collectin dust) and a bmfit. Personally I would never go back. The app IS less user friendly but the overall advantages far out way the fit it. Most of all is accuracy. I don't want to bad mouth fit bit so ill just leave it at that. This is just my opinion but I suggest wearing it 24/7 and eventually you won't even know its there.