Reference: Duroos wa Fataawa al-Hajj – Volume 1, Page 12, No.4
Question: [Whilst in the state of ihraam], what is the ruling regarding covering the head during sleep?
Response: He must not intentionally cover his head; however, if whilst asleep or drowsy he covered it without intending to, then he must take it off as soon as he wakes up or remembers – and thereby he is not required to pay any penalty.