It sure is possible to have a dream within a dream, though usually, people dream they wake up from a dream into another dream (without dreaming of falling a sleep before the inner dream). When people become aware that they are dreaming, also known as lucid dreaming, one of the minds tricks to get them back into believing that the dream is reality is to make them dream that they are waking up from the dream while in fact they are still dreaming. Slowing down time and going into limbo like in the film inception does not really happen and has been invented for the sake of the film. The rate of time passing by in a dream within a dream is the same as in a normal dream.