You can put it where you sleep away from the fireplace so it will wake you up when there is a greater amount of smoke. Better safe than sorry.
Honestly, I've slept through fire alarms (when I lived in dorms, we had a fire drill in the middle of the night and I didn't even know it happened until people asked me the next morning where I was the previous night). And, if the sirens on fire trucks don't wake you, I'm not sure a smoke detector will either.
I have smoke detectors, and I know they work because the one in the hallway goes off every time I fry something in the kitchen, even when I can't even see any smoke and nothing is burning yet. :grumpy: Still, I'm not so confident they would wake me up if I was sound asleep. I've slept through alarm clocks too (and if I hear the alarm, my reaction would probably be to try to hit the snooze button). Every little noise drives me nuts when I'm trying to fall asleep, but once I'm really asleep, a bomb could detonate in my bedroom and I'd sleep through it. On Christmas Eve, I fell asleep on the couch watching a movie and Santa put out the presents and assembled a basketball hoop only about 6 ft from me and I never woke up...I was told that even my nephew stirred in bed at the other end of the house with the noise of that process, but I never heard a thing!