I have been wondering about this....because I have discovered that if I sleep longer than 5-6 hours, I wake up with a mind-numbing "brain fog" - unable to function for over an hour. It is as if the messages are not being transmitted from "the little grey cells" to my limbs, and even speech is a struggle.
However, if I don't sleep at all - or just have short "cat naps" - then I am able to function reasonably normally - apart from the pain and cracking of the joints. As I am now retired, I can make up some of this sleep deprivation during the daytime - so don't feel abnormally tired.
One of the frustrations of this condition is that we cannot plan anything for certain. I'm always having to cancel appointments/days out. However - if there is something really important in the early morning (like an air flight) I just keep awake the night before, and make up the time later.
The Physio doesn't approve of this - and suggests that I try to re-set my "body clock" by not sleeping during the day. However, I managed this for over 5 years when working night shifts - so what is the difference?