What Causes Excess Sleeping During Medication For Depression?

hi I sleep for 15 plus hours everyday and don't wake up throughout the night. I go to bed say 10pm and will get up at 2 or 3pm. What is wroing with me? I am 36 yrs old. Weight is 70kgs and 165cms. I am taking anti depressants Effexor 75mgs been on them for 10 mths and duromines 40mgs and half a stilnox to get to sleep.