What is the interpretation of the obligation for a pilgrim (hajj) to uncover his head immediately upon waking if he had covered it during sleep, and why is he not permitted to cover it when preparing for sleep?

It is forbidden for a pilgrim in the state of ihram to cover his head. If he covers it forgetfully or while asleep, there is no penalty upon him. However, once he remembers and becomes aware of the obligation to uncover his head, if he continues to cover it, he commits a sin and the fidyah (expiatory sacrifice) becomes obligatory upon him. As for someone not in the state of ihram, there is no harm in him covering his head in any circumstance.