I don't think a coffin is conducive to a good night's rest.
Unless you're meaning was that Barnabas might still sleep in his coffin while he was human, shouldn't that be "day's rest"?
And according to much vampire lore (and more or less the vampire lore DS goes by), the vamp is dead during the day, so presumably all he/she can do is get rest - well, at least in the dead sense of the word.