idle slumber; lazing in bed
Refers to sleeping or dozing when one should be active, often with a nuance of laziness or self-indulgence. Commonly used in the phrase 惰眠を貪る (だみんをむさぼる).
I indulged in idle slumber on my day off.
Extended figurative use: a state of laziness or stagnation, not necessarily literal sleep. Often used in critical or self-deprecating contexts.
He had been inactive for a long time, but finally got himself moving.