Assalamu Alaikum. I have been performing Tahajjud prayer and continuing it. But there are some nights when I feel very tired and sleepy when I wake up yet I pray Tahajjud prayer because as far as I know that one should not miss it once he starts to continue it. Is it okay to sleep if I feel very tired, and should I perform qaja if I don't wake up.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Voluntary night prayer (Qiyaam Layl or Tahajjud) is one of the most honorable acts of worship and greatest acts of obedience, and observing it is one of the characteristics of the devout and the signs of righteousness. If what you experience is mere laziness and slackness, you should resort to self-discipline and strive against your ‘self’ to get rid of such laziness. Do not give in to the devil’s enticing to abandon worship. You should rather continue to perform good deeds, for the most beloved deeds to Allah, The Exalted, are those which are regular and constant, even if they are few. The Prophet, sallallaahu ‘alayhi wa sallam, said to ‘Abdullah ibn ‘Amr : “Do not be like so-and-so who used to perform the night prayer and then stopped performing it.” [Al-Bukhaari and Muslim]

But if what you experience is tiredness that prevents you from attaining presence of heart, concentrating in prayer, and reflecting on the verses of the Quran and might even drive you to make a mistake because of drowsiness, then, in such a case, you should rest for a while and then resume the night prayer after recharging and regaining your energy.

The one who wants to perform Tahajjud prayer should get enough sleep at the beginning of the night to be able to get up active and ready to perform the prayers. He should not overburden himself with what he cannot endure but should rather take it easy upon himself in this regard until he gets accustomed to worship and it becomes easy for him. It was narrated on the authority of ‘Aa’ishah that the Prophet, sallallaahu ‘alayhi wa sallam, said: “If one of you feels drowsy while performing prayer, he should lie down until his slumber has gone, because if one performs the prayer while drowsy, he does not know (if he prays in such a state) he might curse himself (by mistake) instead of asking for forgiveness.” [Al-Bukhaari and Muslim]

It was also narrated on the authority of Anas ibn Maalik that the Prophet, sallallaahu ‘alayhi wa sallam, said: “One should pray as long as he feels active, and when he gets tired, he should sit down.” An-Nawawi said in Sharh Saheeh Muslim: “The Hadeeth urges moderation in worship and forbids overburdening oneself in this regard. One should start his worship when he is active, and if he felt tired or bored, he should rest until this feeling is gone.” [End of quote]

If someone fails to perform the night prayers that he regularly observes, it is recommended for him to make up for them in the following morning between sunrise and Zawaal (when the sun moves away from its central meridian, shortly before Thuhr prayer). It was narrated on the authority of ‘Aa’ishah that when the Prophet, sallallaahu ‘alayhi wa sallam, slept and missed his regular night prayers, he would offer twelve Rak‘ahs in daytime. [Muslim] It was narrated on the authority of ‘Umar ibn Al-Khattaab that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Should anyone fall asleep at night and fail to recite his Hizb (a portion of the Quran that he recites every night) or a part of it, if he recites it between the Fajr and Thuhr prayers, it will be recorded for him as though he had recited it during the night.” [Muslim]

Allah Knows best.