I always have stiff fingers on waking up. what is the cause?

Fluid retention: This is usually due to fluid retention due to dependency or due to alteration in circulation at night by arm position. There are other causes of this like certain rheumatologic condtions but would have other symptoms and other joint involvement. If this is persistent or of concern, see your pcp or rheumatologist.

Several: The most common causes of hand morning stiffness are early osteoarthritis and stenosing flexor tenosynovitis (trigger finger). When we lay down flat, the fluid from our chest and belly distribute to our hands and feet. Inflammation enhances this affect, and can exacerbate symptoms related to arthritis and tendonitis in the morning.