o que significa a expressão "to sit up and take notice"?

I watched a horror movie one of these days that didn't give me much of a hard time trying to recover from flashbacks. But I remember one time when I was much younger and had the brilliant idea of watching The Grudge. You might think that's not the scariest horror movie around these days, but for a 14-year-old I wouldn't be so sure. After watching it, the slightest noises coming from in and out of the house would cause me to sit up and take notice and I had the hardest time trying to fall asleep. But wait, what do I mean by sitting up and taking notice?

When you do such a thing, you become alert and pay close attention. So every time a cat would walk on the roof of my house for example or anything made of wood would crack, I would sit up and take notice. You can become alert like that for many other reasons. Just imagine how horrifying it would be if you were home and heard some sharp noises of a possible shootout right outside of your house. You could say: a gunshot caused everybody at home to sit up and take notice. That's all for today. Take care!