When one is in an extremely deep sleep, one is sleeping like a rock. Rocks are used in this particular context because they do not fulfill any of the scientific qualifications for being living things: they do not move, think, grow, consume, or expel. They do not react to stimuli. They just sit there and maybe get washed around in the tide or fall down the escarpment. Rocks, therefore, could be seen as being in a very deep sleep.
You will usually have to try very hard to wake up a person sleeping like a rock. It often involves not just making loud noises and shaking them but actually pulling them out of bed and into the shower, where you run cold water on them. Or you can put on "Enter Sandman" and jump on their bed, with them in it. These will usually produce results: the sleeper will be both awake and very cranky.
Deep sleepers are usually the ones who sleep like rocks, but light sleepers may do so on occasion. This probably will have involved an utter lack of sleep within the previous 48 hours, however. As well, deep sleepers do not always sleep like rocks. Sometimes they just sleep like people who are unaware of their surroundings while asleep, but wake up immediately, all cool and refreshed, when they hear the morning alarm.