The most common and most likely answer to this crossword clue is the 9 letter word DREAMLESS
I've seen this crossword clue in Financial Times
D
R
E
A
M
L
E
S
S
Buddy explains!
The straight part
We know that "So deep in sleep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case so deep in sleep = dreamless.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.