I used to be a 12-hours sleeper too! I found that if I did simple streching exercises 15 min daily, it reduced my sleeptime to only 10 hours. The effect appeared about two weeks after starting this regime. Looking back I think that it worked as a lymph cleanse so you might want to research that. Rebounding is apparently most effective. Of course streching removes the tension as well so maybe it allowed me to relax and sleep deeper. Aerobic exercise did not have similar effect.
I am down to about 7 hours now but it happened rather gradually over time so I am not very sure what helped. I would warrant a guess that it had to do with letting go, as I have done a lot of emotional healing last years.