That is "True".
There are various stages of sleep, one of them is REM or Rapid Eye Movement stage, when we enter this stage of active sleep, and our muscles lose tone, making it hard to keep up a seated position. Accordingly, when we rest upright, our necks tend to strain or drop to the other side, which can cause pain. There are other issues as well but this should also be noted that in certain conditions an individual needs to sleep in upright position.
