I have been extremely tired and getting worse over the past few months, in fact so bad that I was threatened with a sleep study. However my consultant decided to test my vitamin D levels and it was low. I am now taking 250mcg/10,000 iu vitamin D3 a day for a month. I certainly feel as though I have more energy. I can get up in the morning and can now stay awake until I get home from work. I also had a Dexa scan and was told that I had ostoepenia so the vitamin D must be helping that.