A good friend of mine recently told me how her baby girl woke up when mama tried to put her down, but slept through the city's tornado sirens. Reminds me of my own child:
(For anyone who doesn't know, the bedrooms in our split-level house are downstairs, while the kitchen, living room, and laundry room are upstairs.)
Jason wakes up when I'm upstairs, texting. He wakes up when I open the fridge door, or when the bar stool I sit on creaks, or when I put on makeup and things make a tiny click noise.
On the other hand, he sleeps through our smoke alarm going off (dinner fiasco). Doesn't even stir when someone rings the doorbell, or when the appliance guy comes to take away our old washing machine and install the new one - loudly clunking the old one down and the new one up the stairs in his metal dolly (or hand truck, as they're apparently called now).
I'm quite certain that it all has to do with what STAGE of sleep they're in, rather than whether or not they're a heavy or light sleeper. When they're in that deep sleep stage - make as much noise as you darn well please. But when they're not in that sleep stage - be careful how you breathe.