- not go to sleep
- to remain awake late into the night
- to be awake past the time you were meant to go to sleep or to be awake past your scheduled bedtime
- Mom made sure we stayed up to welcome our dad home after months of him being on military tour.
- It's not a good idea to stay up when you have to be in school the next day.
- Linda tries to ensure that nobody stays up too late on a weekday night.
- Since the time for my favourite game show got shifted, I find myself staying up just to watch an episode.
- Will you stay up with me tonight?
This phrase was first used in the United States and it is a short form of the phrase "Stay up Late". With the advent of late night TV shows, it became very popular to have people stay awake just to watch the show before sleeping, often times missing their bedtimes. So people frequently said they stayed up late, with "late" referring both to the late night TV and just not going to bed on time. Later the phrase was cut short to just "Stay Up".