It's a popular feature, but yes, you need to wear it at night to work. I do often wear my fitbit at night, though never my real watches, but I loosen the strap a little overnight. I don't know how good the Samsung's sleep tracking is, but I regard the fitbit's as a slight joke: if I wake up but lie still it almost always records me as falling asleep again, even when I know for sure I did not. But I do find the ability to set an alarm which won't disturb my wife handy at times.