why does sleepwalking women and gothic horror go hand in hand
You asking me?
The answer is voyeurism.
Here's the thing about gendered horror: the goddamn eroticism. You can't escape it, people want to get their rocks off even when they're terrified-- especially when they're terrified! It's such popular imagery because of the intimacy of a woman with all her hair let down in only a white nightgown highlighted by the pale moonlight. A nightgown is very innocent in its intimacy, there's nothing inherently sexual about it, but that just gets people even more horny! No structured garments underneath-- she's wearing breeches obviously but shhh no she's totally naked save for some sheer billowing fabric.
EDIT: oh my god blah blah blah "breeches! actually she wouldn't be wearing those!! oh my god, they got it wrong, just shoot them in the streets, your honour!" FINE SHE'S NOT WEARING ANY PANTIES UNDER THERE, GOOD FOR YOU YOU GOT ME.
Gasp! Unprotected purity! I sure hope no dastardly villain tarnishes this woman! (they do. they do hope for that actually)
This isn't a very fun answer, is it? But it's worth saying. Horror explicitly involving women tends to be very visual with plenty of (arguably) sexual imagery. Men get the mindboggling horrors inconceivable to the human psyche while women are limited to being eye candy who faint before their minds can even be boggled (no that isn't a euphemism). Even when the women in question previously had a larger role in the story than "sexy lamp", pop culture will be quick to reframe it in the way that has the most sex appeal.
But like... let's take the woman's perspective: you're in a state of undress and completely unaware of your surroundings. It's dark and anyone lurking around at this time might very well have bad intentions, and they might turn those bad intentions on you. And you'll be blamed for being in the wrong place, at the wrong time, in the wrong dress. Terrifying. And people don't really appreciate the terror of it because... it's pretty, isn't it?
But then Lucy is weeping in her sleep, and Mina is covering her feet in mud for propriety. Because who knows what a man will do to them if he sees her naked feet? They're both cowering in fear hoping a drunken man doesn't take notice of them. Because who knows what he'll do to them if he sees two young ladies out at night? They're sweating from not just exertion but stress, and their messy hair clings to their frightened faces. They cannot tell anyone. Because who knows what toll this night might take on their good reputations?
It's not pretty. There's no see-through dresses (seriously their nightgowns are made of fucking linen, not organza), no flowing locks, no full faces of makeup, just pure society-ingrained horror.
But cis men don't typically understand that horror because they aren't usually victim to it. It honestly makes me sad and angry that the imagery is so prominent (and in such a watered-down and bland "sexy" way) because it reduces the actual horror these two protagonists face to nothing more than an audience's voyeuristic fantasy in which the women are only objects to be gawked at. The danger is reframed as tantalizing and enticing "ooo good girls (unknowingly) being bad in their sleep!" rather than... they could have fucking died. Or worse.
... But I still want to draw my girls (Jonathan, Mina, and Lucy) in cute nightgowns, so I'll bite my tongue.
