Does Uncertainty about Sleep Depth Break Wudu?
Question

I read the answer number 36889  and knew that deep sleep invalidates the Wudu. Sometimes I sleep in the train or in the car and I can’t be sure if my sleep was deep or not. In this case, is my Wudu considered invalid?
Praise be to Allah.If a Muslim does Wudu , this Wudu cannot be ruled invalid unless there is certainty that what invalidates Wudu has occurred . Mere uncertainty - even if this uncertainty is strong – does not mean that Wudu is invalidated. 
Al-Bukhari (137) and Muslim (361) narrated that a complaint was made to the Prophet (blessings and peace of Allah be upon him) about when one thinks that something has happened while he is praying. He said: “Do not stop until you hear a sound or notice a smell .” 
An-Nawawi (may Allah have mercy on him) said in Sharh Sahih Muslim: 
“The words of the Prophet (blessings and peace of Allah be upon him), “until you hear a sound or notice a smell ” mean: until it is known that one of them is present; hearing and smelling are not conditions of Wudu being ruled invalid according to the consensus of the Muslims. 
This Hadith is one of the basic matters of Islam and an important principle of Fiqh, which is that things are ruled to remain as is until there is certainty to the contrary, and mere doubt does not affect them. An example of that is the issue in the chapter in which this Hadith is narrated, namely: If a person is certain that he is in a state of purity and becomes uncertain as to whether it has been invalidated, then he is regarded as still being in a state of purity, and it makes no difference whether this uncertainty arises during the prayer or outside of prayer. This is our opinion and the opinion of the majority of scholars, both earlier and contemporary ones. 
Our scholars (from the same school of jurisprudence) said: There is no difference between a case where his uncertainty is fifty-fifty and a case where he thinks that one or the other is more likely. He does not have to redo Wudu in either case.” 
If he is uncertain as to whether his sleep was deep or not , then his Wudu is not invalidated thereby. 
Shaykh Al-Islam Ibn Taymiyyah (may Allah have mercy on him) said in Majmu` Al-Fatawa (21/394):  
“The kind of sleep in which a person is uncertain as to whether he broke wind or not does not invalidate Wudu, because purity is established and certain, and cannot be removed by uncertainty.” 
For more details, please see this category: Nullifiers of Wudu.
For details more about sleep manners in Islam, please see this category: Sleep Manners.
And Allah knows best.