I know it’s uncomfortable but it is extremely important to follow the captains announcement and fasten your seatbelt when requested or better yet, keep your seat belt fastened at all times when you are seated, especially when you are sleeping.  Turbulence casued by what’s called (CAT) Clear Air Turbulence, air movement that usually cannot be seen, can happen on a clear day and without warning, varies from mild to extreme intensities including an air pocket causing the aircraft to drop hundreds of feet and can make for a bumpy if not scary ride.  CAT is created by a number of different conditions like air moving at different speeds meeting or different air temperatures meeting.    In flight turbulence is the leading cause of passenger and crew injuries and one thing to remember is that most of those who are injured were not wearing their seatbelt.