Why are inflammatory arthritides worse at night and in the morning?

Question

In general, inflammatory arthritides tend to be worse at night and in the morning. For example, the morning stiffness associated with rheumatoid arthritis is classically worse in the morning. This is also true of many other inflammatory conditions that cause arthritis (e.g. psoriatic arthritis, lupus, ankylosing spondylitis, etc.)

What makes the joint symptoms worse at night and in the morning? Is this related to mechanical factors (i.e. inactivity of the involved joints at night) - and if so why would that lead to worsening of symptoms? Or is it due to a circadian rhythm in systemic pro- and anti-inflammatory mediators (e.g. cytokines, cortisol, other hormones)? Or some other factors?

I could not find much to address this question. Further references are also appreciated.

Answer 1

Although the etiology is unknown, we have a bit of an idea of its pathophysiology

Etiology of RA is unknown. Although the impact of genetic factors is obvious, the genetic basis is not sufficient to explain the triggering of the immune insult. - pubmed - ncbi

As you have mentioned, the circadian rhythm of cytokines has something to do with the classical pattern of "morning stiffness".

...inflammatory cytokines, which reach peak secretion early in the morning are major players causing the morning stiffness - PMC - ncbi

The claim of pain relief with joint movement/activity is yet to be explained but there is a study wherein exercise in patients with rheumatoid arthritis is beneficial in terms of muscle strength improvement. (Benefits of Exercise in Rheumatoid Arthritis -ncbi)