Can You Pray When You Feel Drowsy?
Question

I have read the Hadiths of Al-Bukhari in which it says that you should not pray if you feel drowsy, however I wasn't sure how drowsy it meant, so on numerous occasions I have prayed whilst feeling drowsy. This was because I was so tired that I would not have got up for another 7 or so hours, so I wouldn't have prayed on the right day. Do I need to re-read those prayers? (as I did know that it was wrong to read whilst feeling drowsy, but I wasn't so drowsy that I couldn't understand what I was saying.)
Praise be to Allah.It was narrated from Anas (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “If any one of you feels drowsy when praying , let him sleep so that (when he gets up) he knows what he is reciting.” (Narrated by Al-Bukhari, Al-Wudu, 206) 
Ibn Hajar (may Allah have mercy on him) said: “Let him sleep” – Al-Muhallab said: this refers to the night prayer, because the obligatory prayers do not come at times when people sleep, and they are not so long that they will lead to a person becoming sleepy. This Hadith is speaking about a specific case, but because of the general meaning of the wording, a person may also do that with regard to the obligatory prayers, if he is sure that there will be enough time left.” 
An-Nawawi (may Allah have mercy on him) said: “This is general and applies to both obligatory and to optional prayers offered at night and during the day. This is our view and the view of the majority. But he should not delay an obligatory prayer until the time for it is over. Al-Qadi said: Malik and a group of scholars interpreted it as referring to optional prayers done at night, because that is the usual time for sleep. 
An explanation of this is also given in another Hadith: “If anyone of you becomes drowsy whilst he is praying, let him sleep until he is refreshed, because if any one of you prays whilst he is drowsy he may not understand what he is saying and he may attempt to pray for forgiveness but may insult himself by mistake instead.” (Al-Bukhari, 212; Muslim, 786) 
It may be understood that the degree of drowsiness referred to in the text is that where a person is unaware and does not understand what he is saying.
For more details, please see the following answers: 22326 , 67934 , 50180 , 10914 , and 140773 .
 And Allah knows best.