Almost everyone past a certain age recognises the first few minutes out of bed: joints that feel set rather than sore, movement that arrives a step behind the intention. It is so common that it rarely gets described to a doctor at all. That is usually fine, and occasionally the reason a treatable condition goes unmentioned for years.
The clock is the part that carries information
Stiffness that loosens within about half an hour of moving is the pattern typically seen with ordinary joint wear. Stiffness that persists for an hour or more, particularly in the hands and particularly with visible swelling, is the pattern rheumatologists ask about, because inflammatory arthritis behaves differently. Nobody can diagnose themselves on that distinction — but it is the single most useful thing to be able to report.
Rest feels like the answer and usually is not
A stiff joint invites stillness, and stillness reliably makes the next morning worse. Cartilage has no blood supply of its own and depends on movement to move fluid through it. This is why gentle, regular loading tends to outperform protective rest in trials, and why "wait until it settles" is among the more counterproductive instincts in this area.
Strength around the joint matters more than most people are told
For knees in particular, the muscle that supports the joint changes how much load the joint itself takes. Progressive strengthening has a better evidence base for knee osteoarthritis pain than most things sold for it, and it is slow, unglamorous and free.
Weight, but with the arithmetic stated honestly
Load through the knee rises by several times body weight during ordinary walking, which is why modest weight change shows up disproportionately in symptoms. It is a real effect and it is also not available on demand to everyone; presenting it as the whole answer is where a lot of advice loses people.
What the evidence does not support
No food, drink or supplement rebuilds cartilage. Claims to that effect run well ahead of the research, and the trials of the most commonly sold joint supplements have been mixed at best. Something can be worth trying and still not be what the label says it is.
A reasonable place to start
Move early in the day rather than waiting for the stiffness to pass, build strength around the joints that complain, and time how long the stiffness actually lasts for a week. If it runs past an hour, comes with swelling or fever, or arrived suddenly, that is a conversation for your doctor rather than a shelf in a shop.