From the prescribing information for Remeron:
In other words, Remeron will increase your levels of norepinephrine, which tends to keep people awake.
Again from the prescribing information:
This, of course, is what you want. So the question is, how do these two effects balance out with dose? It turns out that the norepinephrine effects of Remeron go up much faster with dose than the histamine increasing effects. In other words, the higher the dose, the less effective Remeron is at promoting sleep.
From Tuning the Brain, p. 256, by Dr. Jay Goldstein:
So in other words, you are taking the standard antidepressant dose of Remeron, which has only modest sedating properties. According to Dr. Goldstein, a much lower dose, such as 7.5 mg, would have a far more sedating effect. I have tried a 7.5 mg dose myself, and have found it to be extremely sedating.
Of course, at the lower doses, the antidepressant effects of Remeron are less pronounced.