臥
ガ, ふせ.る, ふ.せる, ふ.す
bend down, bow, lie prostrate
JLPT0 G13 S0 F5000
As per
臣
# (fix upon) +
人
person (→ human agency) → *lie face down* (fixing the body to the ground/bedding below) → *sleep*.
[Full Kanji Etymology List]