This is the 9162nd most frequent Japanese word.
熟睡
Deep sleep.
熟睡 is used here as a noun describing a state of sleeping deeply.
昨夜は熟睡しました。
I had a deep sleep last night.
熟睡 is again a noun, and here it is being used as part of a descriptive action.
彼は熟睡していて、起こすのに苦労しました。
He was sleeping deeply, and it was difficult to wake him up.
熟睡 is referenced as the desired outcome in discussing the facilitation of sleep.