Some of the Sahaabah (may Allaah be pleased with them) used to do difficult work such as farming, herding livestock, gathering firewood and so on, but they did not neglect their prayers, rather they used to observe the prayers regularly, not just on time but also in congregation. They were regular in praying and seeking knowledge, and they were not granted any concession allowing them not to pray because of work.
Hence it is obligatory for those who work and others to offer the prayers on time. Allaah praises the believers who do not let their work distract them from obeying Allaah, as He says (interpretation of the meaning):
“Men whom neither trade nor sale (business) diverts from the remembrance of Allaah (with heart and tongue) nor from performing As‑Salaat (Iqaamat‑as‑Salaat) nor from giving the Zakaah. They fear a Day when hearts and eyes will be overturned (out of the horror of the torment of the Day of Resurrection).
38. That Allaah may reward them according to the best of their deeds, and add even more for them out of His Grace. And Allaah provides without measure to whom He wills”
The one who sleeps is excused at the time when he is sleeping, but when he wakes up it becomes obligatory for him to perform the prayer after he has woken up.
It was narrated that Anas ibn Maalik (may Allaah be pleased with him) said: The Prophet of Allaah (peace and blessings of Allaah be upon him) said: “Whoever forgets a prayer or sleeps and misses it, the expiation for that is to pray it when he remembers it.” Narrated by al-Bukhaari (572) and Muslim (684).
This hadeeth indicates that the one who sleeps is not accountable during the time of his sleep, and there is consensus on this point.
The apparent meaning of the hadeeth is that there is no negligence in sleep if a person sleeps before the time for prayer begins or after that but before the time becomes too short. And it was said that if a person deliberately goes to sleep before the time becomes too short and he takes that as a means of not praying because he thinks it most likely that he will not wake up until the time for the prayer is over, then he is sinning. But the apparent meaning is that there is no sin on him because of sleeping, because he did that at a time when it was permissible to do it, so he is included in this hadeeth. But if he did it deliberately in order to miss the prayer, then there is no doubt that he is sinning thereby. End quote.
Nayl al-Awtaar 2/33, 34.
What a person should do before going to sleep is to be keen to wake up at the time of prayer, and to take the means that will help him to perform the prayer on time. If he does that but does not wake up, then he is excused because he did what he had to, and “Allaah burdens not a person beyond his scope” [al-Baqarah 2:286]. That happened to the Prophet (peace and blessings of Allaah be upon him) on some of his journeys.
It was narrated that Abu Qataadah (may Allaah be pleased with him) said: “We travelled with the Messenger of Allaah (peace and blessings of Allaah be upon him) by night and some of the people said: “Why don’t you let us stop at the end of the night so that we may rest, O Messenger of Allaah?” He said: “I am afraid that you will sleep and miss the prayer.” Bilaal said: I will wake you up.” So they lay down and Bilaal leaned his back against his mount, but his eyes grew heavy and he slept. Then the Prophet (peace and blessings of Allaah be upon him) woke up when the sun had risen. And he said: “O Bilaal, what about what you said?” He said: “I have never slept like that before.” He said: “Allaah took your souls when He willed and returned them to you when He willed. O Bilaal, get up and call the people to prayer.” And he did wudoo’ when the sun had risen high and turned white, then he stood and prayed. Narrated by al-Bukhaari (570) and Muslim (681).
The Prophet (peace and blessings of Allaah be upon him) took the means of waking up by appointing Bilaal to alert them when it was time to pray, but sleep overtook him, and he and the people slept until the sun had risen. They were not negligent, hence the Prophet (peace and blessings of Allaah be upon him) said, “There is no neglect among the people.”
As for one who stays up all night because of work etc and does not take the means and prays after the time for the prayer is over, he is regarded as deliberately not praying, and he is not excused because of his sleep.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked about someone who stays up all night and is not able to pray Fajr until after the time for it is over – will it be accepted from him? And what is the ruling on the rest of the prayers which he offers on time?
With regard to Fajr prayer which he delays until the time for it is over when he is able to offer it on time, because he could sleep early, this prayer of his will not be accepted from him, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does an action that is not in accordance with this matter of ours will have it rejected.” Narrated by Muslim. The one who deliberately delays a prayer until the time for it is over, with no excuse, will have it rejected, because he has done something that is not in accordance with the command of Allaah and His Messenger, so it will be rejected.
But he may say: I was asleep, and the Prophet (peace and blessings of Allaah be upon him) said: “Whoever sleeps and misses a prayer or forgets it, let him pray it when he remembers it, for there is no expiation for it but that.”
We say: If he could have slept early so that he would wake up early, or he could have set an alarm clock to wake him up, or he could have asked someone to wake him up, then his delaying the prayer and not getting up for it is regarded as deliberately delaying the prayer, so it will not be accepted from him. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 12/question no. 14
A man may be a heavy sleeper, and one of two scenarios must apply:
1 – He sleeps deeply because he has stayed up at night to work, seek knowledge or pray qiyaam. In this case it is not permissible for him to miss performing prayers on time because of that, and he should look for another job that will not cause him to miss the prayers. Similarly it is not permissible for him to let naafil deeds – even seeking knowledge, which is basically obligatory – distract him and cause him to miss the prayers. In this case his not praying is regarded as deliberate, because he could change his job and he could avoid staying up at night.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said:
What our brothers who go on trips should do is thank Allaah for this blessing, as He has made them prosperous and given them a life of comfort and ease, in an atmosphere of safety and freedom from fear, and they should do what Allaah has enjoined on them of praying on time, whether that is Fajr prayer or any other. It is not permissible for them to delay Fajr prayer on the basis that they are sleeping, because in most cases there is no excuse for this sleep, because they can use alarm clocks to wake them up to pray on time, and they can sleep early so that they can wake up refreshed. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (12/question no. 14)
2 – A person may be a naturally heavy sleeper, and it has nothing to do with work or staying up at night. This is known in the case of some individuals. If that is the case, then he is excused if he takes the means but does not wake up.
It was narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with him) said: A woman came to the Prophet (peace and blessings of Allaah be upon him) when we were with him and said: “O Messenger of Allaah, my husband Safwaan ibn al-Mu’attal does not pray Fajr until the sun has risen.” Safwaan was with him, so he asked him about what she had said, and he said: “O Messenger of Allaah, we are a family who are known not to wake up until the sun has risen.” He said: “When you wake up, then pray.” Narrated by Abu Dawood, 2459; classed as saheeh by Shaykh al-Albaani in Irwa’ al-Ghaleel, 7/65
Conclusion: in your case it seems that your sleeping heavily has to do with staying up at night, and you are staying up at night because of work. On this basis, it is not permissible for you to stay in that job, because it is causing you to forsake the most important pillar of Islam after the Shahaadatayn. So look for another job, and Allaah will compensate you with something better than it. You will see a change for the better in your religious commitment, your body and your mental attitude. As for your religious commitment, performing the prayers on time is one of the most important duties, and not doing it is one of the worst of haraam actions. As for your body, the doctors have described many harmful effects suffered by those who work at night; sleeping during the day does not give the body the same rest as sleeping at night does. And all of that has a negative effect on one's mental attitude.
And Allaah knows best.