the word myalgia just means pain so polymyalgia would be pain from different sources.

fibromyalgia is specific for the fibrous tissues
(ligaments, tendons, and muscles)

It's a sleep related syndrome. The body doesn't repair itself because of a lack of phase 4 sleep also known as restorative or reparative sleep.

It's a vicious cycle because once you've got it you're in pain and stressed and it gets harder and harder to sleep, so the condition gets worse.

Many people with fibromyalgia (myself included) have sleep apnea. As we start to slip into deeper sleep, the muscles of the throat relax and the air passage closes up some. Some people just snore, but for others the passage is too narrow and they literally choke. You don't always wake up completely but you stay in a light sleep.
The body doesn't repair itself in that sort of sleep so you get buildups of phosphates, lactic acid and other waste products in the muscle cells.

Another thing that is affected is that the blood vessels become more permeable and leak some substances. This leads to irritated bowel and irritated bladder syndromes.

Finally there is a sort of mind-fog that goes with the rest. It's a combination of simple exhaustion from poor sleep and all the toxins floating around in your system. You wake up exhausted in the morning and by 10 AM you're dead on your feet. But even if you had the opportunity you couldn't sleep. The whole sleep process is out of whack.

Generally speaking it's a real bummer.