I tried several of the cotton/spandex mix light bras, that I thought would work well as either liners under a good support bra, or sleep/relaxing bras.
I liked this one the best.

The only reason I did not give this 5 starts,  is that it is  marketing itself as a light sports bra (and not a relaxing or sleeping bra).
I guess for the small breasted person, maybe it would work as as sports bra.  But then if one does not need any support, almost any bra would work as a sports bra.