The numbing charms can't do enough to dull the pain – she still can't manage full strength charms with Malfoy's wand – and Malfoy has no access to pain potions, so he's not been near comfortable enough to manage any proper, deep sleep. He has a cabinet full of dreamless sleep potions, but while they help make one drowsy and keep nightmares from troubling one's sleep, they can't make someone go to sleep when they're trying hard not to scream and cry from the constant agony.