Being Overcome by Tiredness at Fajr Time
Allaah, the Most High says: O you who believe! Approach not the prayer when you are in a drunken state, until you know (the meaning) of what you utter.  Does this Verse include the situation of those brothers who come to Fajr prayer while they are afflicted by drowsiness? And what advice do you offer to those brothers?
I advise those brothers who come to Fajr prayer when they are very drowsy to go to bed early at night, because if they go to bed early, they will be relieved of this heavy drowsiness, which causes them not to know what they are saying during their prayers. That is the solution to this problem.
 An-Nisa 4 :43.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Islamiyah Vol. 2 Page 386