If you decide to supplement vitD in winter time note that US government recommendations are way too low [1][2] compared to normal practices in Nordic countries: should be daily 3000 IU for kids (toddlers and up) and 8000 IU for adults.
This is anecdotal, but I can confirm this from my experience. I was taking 2000 IU per day, and I felt the effects of seasonal depression. When I increased it to 5000, I felt so much better.
I agree that the recommended dose is probably too low, but part of the reason for the discrepancy may be poor absorption of many supplements. I ended up with side effects (racing heart and thirst that wouldn't go away - which reliably went away when I backed down the dose and came back when I upped it again) from only 2000IU/day (as an adult). But I was taking it in the form of drops under the tongue (and as a combined formulation of D3 and K2) which I suspect is absorbed much better than the typical pills which are swallowed.