Once that grip on him loosened, Belphegor had little trouble getting to sleep and staying asleep.

What that meant was the next twelve (or more) hours slipped by with little effort on his part, all of the time spent asleep and most of it spent wrapped around Leviathan. At some point in the morning, Bel's hold on him was relinquished, possibly thanks to some internal clock that registered when Levi usually attempted to pry him away to climb out of bed.

So, when Bel finally woke, he expected to find himself alone and with a need to force himself out of bed to seek out Levi elsewhere in the house.