Does Sleeping Break Wudu?
Question

What is the evidence that sleep invalidates wudu?
Praise be to Allah.Evidence that sleeping breaks Wudu
With regard to the evidence that sleep invalidates wudu, that is proven by the hadith of Safwan ibn ‘Assal (may Allah be pleased with him) in al-Sunan. He said: The Messenger of Allah (peace and blessings of Allah be upon him) used to command us, if we were travelling, not to take off our khufuf for three days and nights, except in the case of janabah, but not in the case of stools, urine or sleep. (Narrated by al-Tirmidhi, 89; classed as hasan by al-Albani. So he mentioned sleep as one of the things that invalidate wudu.)
Does sleeping break Wudu?
The scholars (may Allah have mercy on them) differed as to whether sleep invalidates wudu or not, and there are several points of view, including the following: 

That all kinds of sleep invalidate wudu, whether it is a little or a lot, no matter in what position one sleeps. This is the view of Ishaq, al-Muzani, al-Hasan al-Basri and Ibn al-Mundhir, because of the hadith of Safwan ibn ‘Assal (may Allah be pleased with him) quoted above, where he mentioned sleep as one of the things that invalidate wudu, and did not define it any further. 
That sleep does not invalidate wudu in all cases, because of the hadith of Anas ibn Malik, according to which the Sahabah (may Allah be pleased with them) used to wait for ‘Isha at the time of the Messenger of Allah (peace and blessings of Allah be upon him) until their heads drooped, then they prayed and they did not do wudu. (Narrated by Muslim, 376) According to the report of al-Bazzar: they would lie on their sides. 

This is the view of Abu Musa al-Ash’ari (may Allah be pleased with him) and Sa’id ibn al-Musayyib. 
These two views are opposite, and each of them is based on a part of the evidence. However, the majority of scholars reconciled this evidence and said that sleep invalidates wudu in certain circumstances and not in others, although they differed in the way in which they reconciled between the evidence. 

If a person sleeps sitting with his backside firmly on the ground, it does not invalidate wudu, and if his backside is not firmly on the ground, it does invalidate wudu, regardless of what position he is in. This is the view of the Hanafis and Shafi’is. Al-Majmu’, 2/14 
Sleep invalidates wudu except light sleep in the case of one who is sitting or standing. This is the view of the Hanbalis. See al-Insaf, 2/20, 25 

The reason why an exception is made in the case of light sleep of one who is sitting or standing is that in this case the place where wind is emitted will be joined together and in this case one may think it most likely that he has not broken his wudu. 
    3. Some of them said: a great deal of sleep invalidates wudu whatever the case, unlike a little sleep. This is the view of Malik and was narrated in one report from Ahmad. 
The difference between deep sleep and light sleep
The difference between a great deal of sleep and a little is that a great deal of sleep is deep sleep in which a person does not feel that he has broken his wudu if that takes place. A little sleep is that in which a person does feel that he has broken his wudu if that takes place, such as passing wind. 
This view is the one favoured by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), and among our contemporary scholars it was the view favoured by Shaykh Ibn Baz, Shaykh Ibn ‘Uthaymin and the scholars of the Standing Committee, and it is the correct view. This view reconciles all the evidence, for the hadith of Safwan ibn ‘Assal indicates that sleep invalidates wudu and the hadith of Anas (may Allah be pleased with him) indicates that it does not do so. 
The hadith of Anas is to be interpreted as referring to light sleep in which a person can feel that he has broken his wudu if that takes place, and the hadith of Safwan is to be interpreted as referring to deep sleep in which a person does not feel if he breaks his wudu. 
This is supported by the words of the Prophet (peace and blessings of Allah be upon him): “The eye is the string that ties the anal sphincter; when the eyes sleep, the string is loosened.” (Narrated by Ahmad, 4/97; classed as hasan by al-Albani in Sahih al-Jami’, 4148) 
What this means is that wakefulness is the string that ties the anal sphincter, i.e. it keeps what is inside from coming out, because so long as a person is awake he can feel what comes out, but when he sleeps the string is loosened. 
Al-Tibi said: “When a person is awake, he keeps control of what is in his stomach but when he sleeps he loses that control and his muscles relax.” (‘Awn al-Ma’bud) 
If a person cannot control his sphincter in the sense that if he breaks his wudu he cannot feel it, then his sleep invalidates wudu, otherwise it does not.  (Al-Sharh al-Mumti’, 1/275) 
Al-San'ani said in Subul al-Salam (1/97): 
“The most likely to be correct is the view that sleep invalidates wudu, because of the hadith of Safwan… but the word nawm (sleep) in his hadith is general in meaning, and there is the hadith of Anas which speaks of the sleep of the Sahabah and says that they did not do wudu even if they snored, and that they used to lie on their sides, and that they would be woken up. The basic principle is that they are people of integrity and that they were not unaware of what invalidates wudu, especially since Anas narrated this from the Sahabah in general, and it is well known that among them were knowledgeable men who had deep knowledge of the religion, especially the prayer which is the greatest pillar of Islam, and especially those among them who used to wait to pray with the Prophet (peace and blessings of Allah be upon him). They were the elite of the Sahabah, and as that is the case, the general wording of the hadith of Safwan is to be taken as referring to deep sleep only, in which a person is no longer aware, and what Anas said about snoring, lying on their sides and being woken up is to be understood as referring to sleep that was not deep, because a person may snore at the beginning of sleep, before sleep has become deep, and lying one one’s side does not necessarily mean that one is sleeping deeply.”
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in Majmu’ al-Fatawa, when listing the things that invalidate wudu: 
“A great deal of sleep means if it is such that the sleeper does not feel it if he breaks his wudu. But if the sleep is light and the sleeper can feel it if he breaks his wudu, then sleep does not invalidate wudu. It makes no difference if the sleeper is lying down, sitting, leaning on something or sitting without leaning. What matters is the level of awareness. If it is such that he will feel it if he breaks his wudu, then his wudu is not invalidated by sleep. If he is in a state where he would not feel it if he breaks his wudu, then he must do wudu, because sleep itself does not invalidate wudu, rather there is the probability that one has broken one's wudu when sleeping. But if one is still aware and can be certain that he has not broken his wudu because he can feel it if it happens, then sleep does not invalidate his wudu. The evidence that sleep itself does not invalidate wudu is the fact that light sleep does not invalidate wudu. If sleep did invalidate wudu then a little or a lot of it would do so, just as a little or a lot of urine invalidates wudu.”
Similarly in Fatawa Ibn Baz (10/144) it says: 
“Sleep invalidates wudu if it is so deep that one is no longer aware, because the great Sahabi Safwan ibn ‘Assal al-Muradi (may Allah be pleased with him) said:  The Messenger of Allah (peace and blessings of Allah be upon him) used to command us, if we were travelling, not to take off our khufuf for three days and nights, except in the case of janabah, but not in the case of stools, urine or sleep. Narrated by al-Nasai and al-Tirmidhi; classed as sahih by Ibn Khuzaymah. 
And Mu’awiyah (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “The eye is the string that ties the anal sphincter; when the eyes sleep, the string is loosened.” (Narrated by Ahmad and al-Tabarani. Its isnad contains some weakness, but there are corroborating reports that support it, such as the hadith of Safwan mentioned above, thus it is counted as a hasan hadith.) 
As for feeling drowsy, this does not invalidate wudu because it does not cause loss of awareness. Thus the ahadith that have been narrated on this topic can be reconciled.”
The scholars of the Standing Committee said: 
“Deep sleep may lead to breaking of wudu, so whoever sleeps deeply in the mosque or elsewhere has to repeat his wudu, whether he was standing, sitting or lying down, and whether he has a masbahah in his hand or not. But if the sleep is not deep, such as dozing off, in which one does not lose awareness, he does not have to repeat his wudu, because of the sahih ahadith which have been narrated from the Prophet (peace and blessings of Allah be upon him) concerning that, as mentioned above.” (Fatawa al-Lajnah al-Daimah, 5/262)
And they also said: 
“Light sleep in which one does not lose consciousness does not invalidate wudu. It was narrated that the Prophet (peace and blessings of Allah be upon him) used to delay ‘Isha prayer on some occasions, until the heads of the Companions of the Messenger of Allah (peace and blessings of Allah be upon him) would droop, then they would pray and would not do wud.” (Fatawa al-Lajnah al-Daimah, 5/263) 
(See al-Majmu’, 2/14-24; Mawahib al-Jalil, 1/312; al-Sharh al-Mumti’, 2/189-191)
For more about sleep related issues, please see these answers: 21728 , 12782 , 2355 and 21216 .
For more about wudu, please see these answers: 45730 , 11497 and 10672 . 
And Allah knows best.