Some airlines have pathetic seat pitch. Combine this with some very narrow seats, and problems are inevitable. Seat belts signs can sometimes be on for extended periods in turbulence.
Crew commonly leave the seat belt sign on during the night, to avoid the problem of switching it on / off when passing turbulent areas. This is to avoid the need to wake pax while checking belts are fastened each time the sign is switched on. During this extended sign on time, pax commonly ignore it due to the absense of turbulence. They lie across empty seats without seat belt on, and make toilet visits without permission of the crew.
Individual captains finally accept this situation over time.
The problem is with the airlines. They choose the seats and the pitch. They know about the night time problem. Neither risk has been adequately addressed.