Skellig does have the advantage of extra-enhanced eyesight - he would be willing to rise from the bed and add some more fuel to the fire - but being used as a body pillow means that he is in no hurry or desire to move anytime soon.

He LIKES being used as her body pillow.

(It is a subtle way that he can feel claimed by her, which he desires greatly.)

So instead he lays quiet and still, his only movements that of his fingertips as he lightly traces circles on her skin - it is impossible for him to stay truly still.