Well firstly, the mind has the ability to project and believe stimulus' regardless of the fact that they do not exist whatsoever. You may hear a sound while staying up late at work when really there was no sound at all and the truth behind the hallucination is that you are so bored that you wanted to hear something, so your mind made you hear something (although there was actually nothing).

This is the basis of hallucinations and schizophrenics - their minds project sensory stimulus and believe they are real because of whichever neurosis they suffer. The hallucinations are typically (if not always) corelated to the neurosis of the individual.

Your mother calling your name during the transition to a hypnogogic state before falling into slow wave sleep could symbolise the desire to be at home in the care of your mother rather than being on your own at College. It's a wish-fulfilled hallucinagetic projected stimulus. This can happen easily during "day-dreaming" as you simply become engrossed into vague thoughts or fantasies.

Neurologically, it is simply the increased production of gamma-amino butryic acid (GABA) or suppressed production of n-methyl-d-aspartate (NMDA) and other glucides. Which can explain why those who feel groggy and tired will tend to day dream and drift off into "cataleptic" states although they are, physically, still conscious.

Hope I have been enlightening.