What is the ruling on sleeping on an exposed surface, with no barrier or roof between it and the sky?

If the exposed roof is fenced and there is no fear of harm from sleeping on it, then there is no objection to sleeping on it. However, if sleeping on it would cause harm, or if it is not fenced and there is a fear of falling from it, then it is not permissible to sleep on it, due to the Prophet’s (peace and blessings be upon him) saying: “There should be no harm nor reciprocating harm.”