Hi Catrin! I was just searching TE forums for psoriatic arthritis and came upon your post. I also have psoriatic arthritis which has recently gotten pretty bad and I wanted to find other cyclists who might have it to share experiences and stories. I have psoriatic arthritis in my right index finger down to the carpometacarpal joint. I also have it in my second toe on the left foot, and more recently it has "spread" to both my knees and the left index finger closer to the carpometacarpal joint again. Not nice! I finally got myself to a rheumotologist and I have had to start on Prednisone after total failure on NSAIDs. Not happy, and the steroids are not yet having a huge effect either.
I haven't had the problem with spasms in the hand like you describe though, so I'm not sure where those are coming from for you. It does sound like it could be related to the arthritis since it's worse at night (?). Weird though? Have you had any luck sorting it out as yet?
Can I also ask how your body copes with the cycling? I don't know how severe your arthritis is or how widespread, but I haven't ridden in ages and am afraid to get on the bike and flare things upI was an avid enduro MTB'er before this all started going downhill a few months ago and it's killing me being off the bike. Would love to hear from you!