Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "When one of you stands up for offering voluntary prayer at night and finds it difficult to recite the Qur'an accurately, and he is unaware of what he is reciting, he should lie down."
If someone is offering voluntary night prayer, and it becomes hard for him to recite the Qur'an because of drowsiness, until he does not understand what he is saying: then let him lie down until his drowsiness goes away, so that he would not alter or change the words of Allah, for perhaps he might fall into prohibition by reversing its meanings or altering its words, or he might make supplications against himself. In Al-Bukhāri, Anas (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If one of you becomes drowsy during prayer, then let him sleep until he knows what he is reading."