@juergen_hubert@mementomori.social Quite literally - the word for nightmare is painajainen, a being that presses, from painaa, to press. Just like the Scandinavian mara, it could "ride" cattle and horses. There was usually some bad intention behind it: an envious neighbour or a dead person with a grudge.
In folklore, the nightmare is chased away by reciting its origin story. It doesn't have a predictable shape, but it's often associated with flashing light and elements like fire, water and wind. Others say that it can be seen as a dark shape that escapes when you go into the cowshed or stable at night. Sometimes the nightmare is called dignified names in the origin stories, but often it is insulted - to make it obedient and go far away to bother the household of some evil being instead.
Since the Finnish nightmare is so hard to grasp, I got a bit carried away and found a lot of fascinating cures in the main database for Karelian, Ingrian and Finnish folklore:
https://aineistot.finlit.fi/exist/apps/skvr/index.html
It would be fun to know if these cures are similar to sleep paralysis cures in German folklore and elsewhere! (I don't mention here the cures for cattle and horses, although they are also quite interesting, because they seem to suffer from something else than sleep paralysis.)
One common idea seems to be that the nightmare is caused by having a fright, so it has to be expelled by causing a fright - for example, by braiding a cord with fibers from three hemp stalks and three strands of hair from the suffering child’s mother. The braiding has to be done in a sauna and the braider announces that they are making "a noose for the nightmare". The child keeps the cord around the neck until the nightmare has fled.
http://urn.fi/urn:nbn:fi:sks-kvr-068374
Another cure could be to have the child sit on a millstone, turn the mill and (when asked by somebody nearby, "what are you grinding?") exclaim: "The head-presser, the eye-turner!"
http://urn.fi/urn:nbn:fi:sks-kvr-027196
Eye-turner: The sleep paralysis nightmare was thought to cause strabismus.
Another interesting cure is from Satakunta in south-western Finland: the person afflicted by the nightmare can immediately after waking up cure themselves by putting on a piece of clothing of the opposite sex, in the opposite way (e.g. trousers over the head), and then confront the nightmare in a lucid dream.
http://urn.fi/urn:nbn:fi:sks-kvr-058022
The last cure (from Karelia) is about sleepwalking rather than sleep paralysis, but it's just so fascinating that I had to include it - particularly because the people/forces of the forest are involved (metsän väki, the supernatural forces of the forest that can also cause an affliction):
"If someone dreams badly, so that they rise up while asleep and walk around unconsciously, they have to be fed wild reindeer brains unknowingly. On the same Thursday nights, put a female dog's whelping bed under their head without them knowing. Then they will wake up as soon as the dream befalls them. But you have to keep an eye on them, if the people/forces of the forest begin to bother them. This can easily become a bother when one is fed wild reindeer brains. But if they always wake up when they try to sleep, and see strange people in their dreams, you must not let them be without a cure for three nights, or they will go mad and only somebody that was made a sage in his or her mother's womb can cure them. But anyone can cure them before three nights are over. Just take water and wash their head and take them to the place where the wild reindeer was killed, the brains of which they were fed. If you figure out that female forest spirits bother them, women have to do the washing, if the spirits are male, men will do. But whoever does the washing and takes the water into the forest, they have to take three coins of three kings and whittle three slivers, one from each coin, as a sacrifice to the forest spirit, and say:
These are the eternal golds of the moon,
The silvers above the sun etc.
Or if you don't have money, give three drops of blood from the nameless finger [the ring finger] on the left hand."
http://urn.fi/urn:nbn:fi:sks-kvr-004498 
