I am on vitamin b12 and vitamin d3 medication from last 2 and half months ...and even now I am felling following problems 1). Most of the time while studying I feel much fatigued and sleepy , even if I started studying just after a 2 hrs sleep in afternoon. and studying lying down that time give me comfort..but it also only for not more than 15 minutes.. 2).my normal sleeping time is 12am -7am but if someday I woke up at 8:00 am I feel much weak and like my internal body is heated and my head heavy like something had grasped my head from inside (as in common cold)...