1.
If you ~, you move into a sitting position when you have been leaning back or lying down.
Her head spins dizzily as soon as she sits up.
PHRASAL VERB: V P
2.
If you sit someone up, you move them into a sitting position when they have been leaning back or lying down.
She sat him up and made him comfortable.
PHRASAL VERB: V n P
3.
If you ~, you do not go to bed although it is very late.
We sat up drinking and talking...
= stay up
PHRASAL VERB: V P
4.
see also sit-up