Pausanias, in his Description of Greece mentions that Hypnos (the personification of sleep) was particularly friendly with the Muses:
[Paus. 2.31.3] Not far from the Muses' Hall is an old altar, which also, according to report, was dedicated by Ardalus. Upon it they sacrifice to the Muses and to Sleep, saying that Sleep is the god that is dearest to the Muses.
Pausanias does sometimes take care to reference his mythological sources. For example, when he mentions that Hypnos and Thanatos (the personification of death) are brothers, he identifies the Iliad as the source of the tale. Unfortunately, that isn't the case with the relationship between Hypnos and the Muses.
Do we know of a tale that would explain how Hypnos came to be the dearest to the Muses?