my friend wore a smart watch to bed, when he woke up in the morning it showed 300 steps taken how did this happen?

Your tracker will detect your arm movement and will interpret it as steps so this is what it likely happening. Something similar happens while driving, if you go through a bumpy road or your car has a stiff suspension then this can cause the tracker to detect movement and count it as steps. Usually while driving and if you notice some vibration on your car or steering wheel you can try having your tracker a little bit more tight around your wrist and this will help decrease the amount of steps detected.

Explanation:

(Via AlvaroFitbit)