sit_up
1. [ verb ] prevent from going to bed at night
Synonyms: keep_up stay_up
Examples:

"The anticipation of the trip kept the children up all night" "I kept myself up all night studying for the exam"

Related terms: wake
2. [ verb ] not go to bed
Synonyms: stay_up
Examples:

"Don't stay up so late--you have to go to work tomorrow" "We sat up all night to watch the election"

Related terms: wake
3. [ verb ] change to an upright sitting position
Examples:

"He sat up in bed"

Related terms: change_posture sit-up
Similar spelling:   sit-up
  set_up
  sit_by