Sleeping Beauty Disorder
The sleeping beauty disorder is a rare mutation that causes a norn to enter an "enchanted" sleep after a certain point in its life. Norns with this disorder may sleep from childhood until death, with some individuals sleeping for hours on end, never waking. They typically starve or age to death. The up-side of this mutation is that it does not easily grow out of control like some mutations, as sleeper norns obviously do not breed, and instead sleep til death occurs.
It does seem to be genetic, as it runs in families, assuming of course, one can get a sleeper norn to breed before it falls into its eternal coma.
A possible cause for this mutation is that when the Norn sleeps, their sleepiness/tiredness is not affected, and they therefore get no satisfaction from sleeping, despite the Norn's body being programmed to do so.