It was narrated from Anas that the Prophet (peace and blessings of Allaah be upon him) said: “If any one of you feels drowsy when praying, let him sleep so that (when he gets up) he know what he is reciting.” (Narrated by al-Bukhaari, al-Wudoo’, 206)
Ibn Hajar said: “Let him sleep” – al-Muhallab said: this refers to prayer at night, 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 hadeeth 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.
Al-Nawawi said: this is general and applies to both obligatory and to naafil 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-Qaadi said: Maalik and a group of scholars interpreted it as referring to naafil prayers done at night, because that is the usual time for sleep.
An explanation of this is also given in another hadeeth: “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 to pray for forgiveness but may insult himself by mistake instead.” (al-Bukhaari, 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.
And Allaah knows best.
www.islam-qa.com