I constantly get asked about supplements and what things to take, Vitamin D is without a doubt a very important vitamin that everyone should consider. This vitamin has been shown to be very important in hundreds of biochemical processes in the human body that affect many different things from performance to cancer prevention, and mood and sleep. I highly reccomend getting your vitamin D tested to see if supplementation will benefit you, But here are some research article summaries, kind of dry reading, but i highly recommend reading them. They are short but have great info.
http://www.vitasearch.com/get-clp-summary/40688
http://www.vitasearch.com/get-clp-summary/40618
http://www.vitasearch.com/get-clp-summary/31109
http://www.vitasearch.com/get-clp-summary/40647
http://www.vitasearch.com/get-clp-summary/40584
http://www.vitasearch.com/get-clp-summary/40532