Crossword Genius

Stayed awake late, prepared to take notice (3,2)

Ross

I believe the answer is:

sat up

I'm a little stuck... Click here to teach me more about this clue!

'stayed awake' is the definition.
(I've seen this in another clue)

'late prepared to take notice' is the wordplay.
I cannot quite understand how this works, but
'prepared' could be 'up' (similar in meaning) and 'up' is located in the answer.
The remaining letters 'sat' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for sat up that I've seen is " Suddenly became alert".)

I've seen this clue in the Evening Standard.
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