A sleep you could not fight is described as a gift
If one thing in this course reaches you, let it be the first of these.
There is a sound report that a man who habitually prays at night, and is then overcome by sleep, has the reward of his prayer written for him, and his sleep is described as a charity given to him. Not a debt. Not a failure. A charity, to him.
And the second is the tradition telling a drowsy worshipper to go to bed, because someone praying half asleep does not know whether they are asking forgiveness or cursing themselves. The instruction, in that state, is sleep.
Qur’an and Sunnah
It was narrated from Sa'eed bin Jubair, from a man who he thought was good, that :Aishah, may Allah (SWT) be pleased with her, told him that the Messenger of Allah (ﷺ) said: "There is no man who habitually prays at night, then sleep overwhelms him, but Allah (SWT) will record for him the reward of his prayer, and his sleep is a charity given to him
Sunan an-Nasa'i 1784graded sound, albanifull report
Narrated `Aisha:Allah's Messenger (ﷺ) said, "If anyone of you feels drowsy while praying he should go to bed (sleep) till his slumber is over because in praying while drowsy one does not know whether one is asking for forgiveness or for a bad thing for oneself
Sahih al-Bukhari 212graded sound, sahihaynfull report