No time to sleep, or if you forget to pray, and when its term expires, then you will feel when or if he will be conscious of the (obligatory) prayers, reading is made up.

Prophet (peace be upon him) said, "Whoever is no prayer is forgotten or falls asleep, his atonement is a reminder of just how to read." In another narration he said, "It's no atonement (expiation) does not exist." ( Bukhari, Muslim, Mishkat No. 603)

Three said, "There is a state of sleep is no looseness. I was remiss in waking state. So when one of you forgets a prayer or fall asleep, then he should be recalled to just read it. Why, Allah says, "And you're reminding me to be constant in prayer." (Quran 2014, Muslim, Mishkat No. 604)