The preponderant opinion in this regard is that
sleeping which causes a person to lose the senses invalidates Wudu', whether standing, sitting or lying down. However, dozing while one retains full awareness does not invalidate Wudu'. Accordingly, we can reconcile between the Hadiths reported in this regard, among which is the Hadith narrated by
Safwan ibn `Assal (may Allah be pleased with him) who said that:
The Prophet (peace be upon him) ordered them that they should wipe over Khuffs (leather socks) on a journey for three days unless they are in a state of Janabah (major ritual impurity related to sexual discharge). However, in case of defecation, urination, and sleep, they are permitted to do so.
The Prophet (peace be upon him) considered sleeping like defecation and urination and did not differentiate between them.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.