Unless it's the active D vitamin, it takes weeks to convert it into reserves.

Taking vitamin D once, right before bed will do nothing. Our bodies don't use it that way.

Cholecalciferol = weeks

calcidiol = hours

The conversion period strongly depends on the input dose and on available cofactors. Numerous people have reported that 5000 IU of vitamin D3 can produce energy benefits in just a few hours if one has been skipping taking it. If one is low on cofactors, and has a low dose of D3, then I imagine it taking days or even longer as you noted.

> After ingestion, cholecalciferol is first converted to calcidiol in the liver, which can take several hours. Then, calcidiol is further converted to calcitriol, the active form, in the kidneys, which may take additional time.

read as if you're perfectly healthy, at minimum it's 24 hours. What you're talking about is probably placebo effect. For reserves to start building up to the point where your body has "enough" calcitriol, it takes weeks

Imagine the following variables:

* CalcitrolLevel: float

* CalcitrolReserveCapacity: float

* Is_Energetic: bool

Imagine the following equation:

* Is_Energetic = CalcitrolLevel > 90% of CalcitrolReserveCapacity

For most people, it will take days, weeks, or even months to come above 90%. But for those who have dipped just below 90%, such as to 89%, if they're optimally healthy in other ways, they will see a rapid rise in energy by high dose D3. If it's not this, then there is something else going on physiologically to explain the subjective effect.