Is it permissible to delay prayer when feeling drowsy or extremely tired, while still being aware of what is being said?

It is not permissible to delay the prayer beyond its prescribed time, except for a valid excuse. Severe fatigue and drowsiness are not considered valid excuses for taking the prayer out of its time. It is permissible to delay the prayer until the middle or end of its time due to severe fatigue or illness, to enable performing it with devotion and tranquility. It is also permissible to combine two prayers, according to some jurists, for one who experiences hardship by not combining them. However, with severe drowsiness that causes the worshipper not to comprehend what he is saying, it is not permissible for him to begin or complete the prayer until he understands what he is saying.