We have the hadith
... Whenever you go to bed perform ablution like that for the prayer ... -- Sahih al-Bukhari 247 (sunnah.com); see also Sahih Muslim 2710 a (sunnah.com)
However, sleeping (or, at least, deep sleep) is regarded as invalidating wudu (see e.g. Islam Q&A, Darul Ifta Birmingham). Hence the question:
Question: If sleeping invalidates wudu, why are we encouraged to do wudu before sleeping?
Sahih al-Bukhari continues "...if you die on that very night, you will die with faith..." and Sahih Muslim instead says "...you would die upon Fitra...". This seems to attribute a high value to an invalidated wudu, and it makes me think any prior wudu should suffice. But I feel like I'm missing something here.