Dr. William Davis, whom I follow for his cardiology related advice, also advocates taking vitamin D for optimum heart health. He notes that most people lose the ability to synthesize vitamin D as they age. Here's a typical post on the subject:

http://heartscanblog.blogspot.com/2010/01/getting-vitamin-d-...

My own report is that after taking 6000IU/day of vitamin D, I rarely suffer from afternoon drowsiness, whereas before it was so bad I'd sometimes go take a nap in in my car in 90+ degree heat around 3pm.