Just want to know if sleeping xp boost affects idiot savant or are they seperate?

It should.

Feeling fully rested from sleep should gave everyone a XP bonus whether you are genius or dumber than a door knob.

My question is if sleeping affect idiot savant, meaning when you get the idiot savant xp is it bigger because of sleeping or is the idiot savant always the same?

Anyone can get the Fully Rested bonus after sleeping. This bonus will also stack with the Idiot Savant bonuses. The same is true for Lover's Embrace. It'll stack with Idiot Savant (not with well rested though).

Do we know whether each bonus is additive or multiplicative? It'll make a difference in the math.

never noticed...but maybe that's why sometimes I get 100+ EXP. from killing a molerat..

Idiot savant is described as multiplicative. Well rested is additive. It should go something like this....

100 xp gained for an activity becomes....

(xp + well rested) x Idiot Savant Multiplier

(100 + (100 * 0.1)) x 3 = 330

assumes idiot savant multiplier is x3. My understanding, anyway....