The Rosewood pico cage isn't good for a hamster. It isn't just the small size of the base - floor area - it's also a bit tall and has a plastic house at the top.
Hamsters are basically ground dwellers who like to live in burrows - or a nesting box that mimics a burrow, open underneath and sat on top of the substrate. Their normal behaviours are to hoard in or under their nest and have a toilet they can bury nearby their nest or house.
So with the house at the top your hammy has to come all the way down to the substrate at the bottom to make a toilet area. And then sometimes they just end up peeing in the nest instead and it gets all stinky.
With the house being so small and enclosed as well it needs cleaning out a lot and having their nest taken away stresses them. Cleaning generally stresses them as well.
The wheel will just be part of the issue. They tend to bar chew when the cage isn't big enough as well.
how long have you had the cage and the hamster. If it's not long and you got it from pets at home, some people have successfully asked for a refund saying the cage isn't suitable - even after it's been used. So you could get another cage and then see if you can get a refund for the Rosewood pico.
The same thing happened to me at first - i got a 3 tier rotostak and it was totally unsuitable. our syrian is now in the barney cage mentioned above
it costs about £50 and has just about everything you need except a few toys. it has a good shelf and a good sized house for a syrian hamster - although it's an idea to paint the wood shelf and the inside of the house with plastikote water based enamel paint which is pet safe. It has an 8" wheel that's quite basic and ok to start with but you already have an 8" wheel. ideally an 11" wheel is better as they run better in it.
She may be using the wheel at night and the novelty has just worn off a bit - but it's probably also due to the set up not being right for her.
if the barney is too big for you to fit in then the alaska cage is about the smallest size you'd want - the base is 82cm by 48cm ish. Room for plenty of substrate. it has a shelf but you'd need a good sized house to sit on top of the substrate. it also has a good big front opening door which helps with taming and interaction. The price is reduced at the moment as well so it's £31.99
https://www.zooplus.co.uk/shop/small...r_cages/284288
For now maybe try and rearrange the pico cage a bit. put as much substrate as you can in the base, without it falling out of the bars and add another house at floor level - a tissue box with a hole cut in would do. i'd also recommend removing the top plastic house if that's possible.
Then get rid of the long walk up to the top - maybe have two of the shelves at the same level as each other and just one ladder to the shelves. So the only reason she has to go up the ladder is to get to the shelf - with a food bowl on it eg. She can then basically live in the base of the cage with the house and wheel.
if you upgrade her cage, it doesn't completely stress her out. Although it's nicer for them with a new cage, they need to adjust as any change of environment is stressful. i tend to do a transition over a day or so where they just play in the new cage, partly set up, before moving in. And move everything over from the old cage without cleaning anything so it smells familiar.