Insane people can summon the strength to do things that seem physically impossible. I've never seen this in action, but I believe it: they don't worry about the consequences, and put their heart and nerve and sinew into whatever it is that they feel they must do. And Millicent felt she must put up a facade of being normal - or anyway, her crackpot version of normal.
Really? I just awakened and already I've learned something. Well explained too. Adrenaline I suppose, though I never understood how that could make you stronger.
FWIU, insanity (also alcoholism, drug abuse, etc.) provide only the
illusion of increased physical strength and intellectual function. Lydia, you may be thinking about "temporary insanity" or rage which, as MT said, causes the body to release adrenaline (epinephrine) and norepinephrine. Other hormones are released, but these in particular cause the increased physical strength and motivation, a heightened perception, and extend the person's capacity to function, plus and other symptoms. Bipolar disorder and other conditions (can you say PMS?) can increase the potential to become enraged, but the release of epi and norepi is still in reaction to the anger or extreme stress.
Sorry for going off topic here.
