Fall, a high level of serotonin is not necessarily serotonin syndrome, you will be delighted to know !
Sometimes it is not because you are responding badly to SSRIs or other medications, but rather just because...life is good and your body has gotten "into the groove". Maybe you have just learned to "chill" ?
Even on the roughest river we sometimes meet a smooth, still patch where we can catch our breath and just enjoy watching the scenery go by before we have to paddle like H*** again...
My (off the cuff, totally amateur, so who knows how naff) guess is that the Vitamin D is keeping your body topped up with the optimal levels of serotonin, giving you that lovely snuggled-up-warm-just-awake feeling even when you are not, and probably more so in the evenings. If your sleep pattern is not improved by this I'd say that this is because your pineal gland already produces melatonin when and as it should, which is a good sign. (Wish mine would, I wouldn't have SAD !
) If this is a side effect associated with 6-MercaptoPurine, I want some !
Good to know you are feeling fine, whatever the reason...