p/s/a: Vitamin D supplements can take you from depressed zombie to functioning human being in one week.
Anecdote time!
A therapist suggested a few months ago that I get my Vitamin D level checked — citing low levels as a possible cause of low, somewhat depressive moods. I finally got around to doing that. Last week I got the test results; it turns out it's a bit below the normal range. So my doctor advised I should take 1000 IU of Vitamin D3 daily. I'm not sure it's working, but I have generally felt both more cheerful and more productive since.
But don't start taking it without actually testing first. Hypervitaminosis D doesn't sound like any fun at all.
You need to take more than 4000 IU/day (probably for quite some time) to have any hope of toxicity. I knew a man who accidentally took 50,000 IU daily for one week (meant to be weekly per his prescription) and suffered no ill effects. Some people may even require doses above 4000 IU/day to get their Vitamin D levels up to target.
P/S/A: There are single sentences which can create life-changing amounts of difference.