Constant drowsiness and Fatigue. Done.

I am a 24 year old male working as a tax analyst for a corporate. My working hours are approximately 10 hours a day, I am a regular jogger and do simple exercises everyday, Over the last 3-4 years I have been experiencing constant drowsiness and Fatigue throughout the day, This is affecting my work and personal life badly. Am unable to focus and give my 100% for any task I do. I end up feeling dull and lazy through out the day. I met my Family Physician 4 months ago and explained him what i had been going through. He suggested a complete bloodcheck up and the test was done for the same. The results were fine and normal except low score for vitamin D of '18'.The doctor suggested me to take Vitamin D360 capsules once in a week to be continued for upto 6 months. I have been taking this medicine and find no difference at all. For any physical activity done I end up sluggish and exhausted with body aches. Kindly request you to advise on further actions to be done.