Quran 73:20

[Prophet], your Lord is well aware that you sometimes spend nearly two-thirds of the night at prayer––sometimes half, sometimes a third––as do some of your followers. God determines the division of night and day. He knows that you will not be able to keep a measure of it and He has relented towards all of you, so recite as much of the Quran as is easy for you. He knows that some of you will be sick, some of you travelling through the land seeking God’s bounty, some of you fighting in God’s way: recite as much as is easy for you, keep up the prayer, pay the prescribed alms, and make God a good loan. Whatever good you store up for yourselves will be improved and increased for you. Ask God for His forgiveness, He is most forgiving, most merciful