One of the things I emphasize during my Home Alone class for children ages 8-10 years is that thy should not be left home alone when they're sleeping. They sleep so soundly that their brains won't register that the sound they're hearing is the smoke alarm.
WCCO did a segment on this last night focussing on the importance of practicing fire drills with your family at different times of the day and night. However, I'd argue that even with a great deal of practice the brain may still not register what the sound is when the child is in a deep sleep. There have been numerous studies done on this which of course raises much concern for parents, firefighters and those who make smoke detectors.
It's still important to practice and have a plan of action with your children in case of a fire day or night. However, as the adult(s) in the household you need to realize that children's sleep is different than adult's and ultimately we are still responsible for making sure they wake and get out when there's a fire. I strongly discourage you from leaving your children home alone when they're sleeping.