Whilst sleeping there are four postures one can adopt:
1- To sleep one’s stomach.
It is mentioned in a hadeeth that this position is the sleep of Shaytan i.e he who sleeps this way his sexual desires are aroused and Shaytan can then mislead him into sin.
A companion (RadhiyAllaahu Anhu) narrated: “Once while I was lying on my stomach in the mosque, a man moved me with his foot and said, "This is a position which Allah hates."
He said, "I looked up and it was the Messenger of Allah, may Allah bless him and grant him peace." [Abu Dawud]
2- Sleeping flat on the back.
3- Sleeping on the left hand side.
One should abstain from this because in this condition the heart becomes negligent.
4- Sleeping on the right side with the right hand under the cheek.
This is the Sunnah position. In this position one is not negligent and it will be easy for him to wake up.
“When a Mu’min (believer) acts according to the Sunnah (tradition) of Rasulullah (Sallallahu Alaihi Wasallam), each such deed carries a light with it which illuminates and enlightens the heart of its doer”