A bit of exercise rather than going to bed? (3-2)

I believe the answer is:

sit-up

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'a bit of exercise' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for sit-up that I've seen before include "Physical exercise" , "Exercise done on the floor" , "Exercise designed to strengthen stomach muscles" .)

I've seen this clue in the Evening Standard.

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