WebMD offers one possible reason for what appears to be an oddity: “Research shows that with fibromyalgia, there is an automatic arousal in the brain during sleep. Frequent disruptions prevent the important restorative processes from occurring. Growth hormone is mostly produced during sleep. Without restorative sleep and the surge of growth hormone, muscles may not heal and neurotransmitters (like the mood chemical serotonin) are not replenished. The lack of a good night’s sleep makes people with fibromyalgia wake up feeling tired and fatigued. The result: The body can’t recuperate from the day’s stresses — all of which overwhelms the system, creating a great sensitivity to pain.” This sounds like a very vicious cycle.
Some speculate that by the end of the day our minds are beginning to clear as we focus on resting which makes us more aware of our fibromyalgia pain. However, conventional wisdom dictates that by the end of the day our minds have been flooded with information gathered by our senses and processed in our brains and bodies. Indeed, this is why we must sleep at night so that we can process all we have absorbed and refresh our bodies. Thus, this flood makes us the least clear at night. Compare it to the clarity you have when you first wake each day. What seems like a much more reasonable conclusion is that the body itself is fatigued just from the activities of the day. Therefore, by the evening, it takes far less to stress the muscles and create more inflammation. In fact, while it may seem like an exaggeration, the muscles of fibromyalgia patients might be mildly compared to rigor mortis wherein the muscles are always contracted. Because fibro patients cannot usually relax their muscles.
reference:Why Does Fibromyalgia Hurt More at Night?