How do i tell my boss i need to move to day shift because of my arthritis?

Re: My question:
I know this may sound like a simple question, but the way of constructing the discussion with her is difficult - i don't want to sound helpless, or that i am disabled in some way and thus unable to perform my duties on the job.... What should i say... er - exactly?

I was thinking of something like "request due to medical reasons", but geez that sounds severe!

Background:
I recently discovered i have arthritis (i'm 26/ F). Before this was clearly so, i started working night shift, in a job where i do a lot of typing (and yes, i'm at work now).

Symptomatic issues:
If i am up during the day i have no problems with my hands, but from about 8pm onwards my hands swell up and all my joints (but particuarly my fingers, elbows and hips) become extremely painful and I have difficulty moving them.

Treatment:
Acupuncture works wonders for me, but recently i have been unable to afford treatment. I'm now taking Voltaren (which is okay) and using the Voltaren gel.

The big thing is that i am/ used to be one of the top performers and now my productivity has dropped significantly. It's so frustrating.

Use the 'request due to medical reasons' it can sound severe but it is a medical reason. I think you worded it best in your Symptomatic Issues.
You do work for her and she pays you = its an even exchange. If she wants the work done well she will compromise and make it a workable situation in order to benefit you both.

How about just sitting down with your boss and telling here about your medical problems and asking if you can be switched to the day shift.

Ask your doctor to give you a document to request working in day shift.