Presumably, you'll be going out at night in that state - and that's supposed to eventually put your mind at rest? (10)
I believe the answer is:
drowsiness
'rest?' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for drowsiness that I've seen before include "Dozy feeling" , "Lethargy, somnolence" .)