I grew up in a house of five kids, me being one of them. Having four younger siblings, and being a night person, I'm a heavy sleeper, its the only way I could sleep.
Several years ago, in my teens, my little brother decided to climb on top of the house to get a frisbee or something that landed on top of it. I was "dead to the world" asleep. In the middle of climbing on top of the house, he fell off the oil barrel. The "thud" woke me up. Mom was startled when I darted out of my bedroom and made a run for the back door, but halfway there she heard Vic start crying and was right behind me.
A few years later, the same little brother wrecked his bike to avoid a car, I think the car was going a little too fast, and since this is Vic we're talking about, I know he was. He wound up going over the handlebars onto the asphalt, knocked out several teeth, had road rash on his face, shoulder, palms and forearms, I think he landed on his face and shoulder, and while sliding he tried to push his face up out of the asphalt (I've done it myself, I was about the same age too). I was asleep that day and woke up.
I could sit here and tell stories all day about times I was dead asleep, but woke up, on my own, when needed. About half of them would involve my accident-prone brother Vic, with the other half about my three other little brother and my little sis, along with people sneaking around the house.
Due to past experainces, I'm pretty sure I'll wake up in the event that a loved one is injured or someone tries to break in.
One thing I've learned is that if you ever wake up, and you're wide awake for no reason, take a look around. There might be something wrong. If you're luck is anything mine, there probably is.