Reward of prayer varies according to the hardship involved
2-5-2017 | IslamWeb
Question:
I work the night shift from 10:00 pm to 10:00 am and I pray Fajr at the mosque as I stay up all night because of my work. Is my reward of the Fajr prayer equal to that of someone who works during the day time and sleeps at night then wakes up at the time of Fajr specially to offer the prayer at its due time although he works all day? Another question; I combine the Thuhr and ‘Asr prayer with the Maghrib prayer because I work all night and sleep all day, is it permissible to do this?
Answer:
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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The reward for an act of worship varies from one person to another if it involves more hardship for one than the other. ‘Aa’ishah 
narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, “Your reward is equivalent to the amount of your fatigue.” [Muslim] There is a difference between a person who wakes up from his comfortable bed to stand up in prayer and another who was not asleep and finds no hardship in it.
Combining two prayers because of sleep is impermissible. Rather, it is obligatory for you to wake up and perform the prayers at their due time and then go back to sleep if you need to. Likewise, a person who works during the day time and perhaps goes to bed late is required to get up to pray the Fajr at its due time. Please know that if you strive against yourself so as to get up to perform Thuhr and ‘Asr prayer at their due time your reward will increase in proportion to the hardship that you suffer. Moreover, we notify you that it is not valid to combine the Maghrib prayer with the ‘Asr prayer whether in a journey or on account of any of the excuses that allow the combination of prayers. The Maghrib is combined with ‘Isha’ only whenever an excuse that allows combination exists, such as travel or rain.
Allah knows best.