My nephew was diagnosed with a Vitamin D deficiency last year at the age of 17. He played hockey for 10 years, so most of his time was spent indoors either in school or on the hockey rink. The doctor prescribed SUNLIGHT for treatment. Seriously, he had to go outside for 15 minutes a day, every day. So he would go out every afternoon and do his homework in the yard.
He was extremely tired, falling asleep in school, and he's a really good kid. We knew there was something wrong with him, not just a kid making excuses. It worked for him.