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.