could be tendonitis, repetative strain injury, carpal tunnel syndrome. But sometimes if it's none of the above, it could be how you hold them.
Position of your neck, the kind of pillow you use, if your sleeves on your nightwear is too tight...etc.
Do you have problems with your hands otherwise? If you have a job that can cause repetative strain injury, neck/shoulder problems, etc...you should see a specialist to help you gain comfort.
It might be as easy as changing position at night, or wearing wrist braces. But to be properly diagnosed..it's possible you'll need surgery if nerves are impinged.
q
