If someone misses a Prayer unintentionally, that is for a legitimate excuse, such as forgetting or sleeping, though he is basically very keen to perform Prayers on time, he is excused and is not sinful. But he would need to make it up as soon as he remembers it. The evidence for that is the hadith narrated in Sahih Muslim which tells how the Prophet (peace and blessings be upon him) and his companions slept and missed Fajr Prayer, and the Sahabah (the Prophet’s companions) began to ask one another, “What is the expiation (kafarah) for what we have done by neglecting the Prayer?” The Messenger of Allah (peace and blessings be upon him) said. “Sleeping does not mean that you are neglecting the Prayer. Rather neglecting the Prayer means not praying one Prayer until the time for the next Prayer comes. Whoever does that let him pray when he remembers it.” Thus, the expiation for missing a Prayer for a valid reason is to make it up once the excuse is over.

This does not mean that a person may sleep deliberately when a Prayer is due, until he misses it, then uses sleep as an excuse, or neglect a means that would help him to perform the Prayer in time, and then take that as an excuse. Rather one must make use of all the means he can, as the Messenger (peace and blessings be upon him) did in this case, when he appointed one person to stay awake and wake them up to pray, but that person was overcome by drowsiness, so he did not wake them up. This is the case in which a person may be excused.