Physiologically, most muscles relax during non-rapid eye movement (NREM) sleep and become atonic (i.e., exhibiting a lack of muscle tone similar to paralysis) during rapid eye movement (REM) sleep, except for the ocular muscles and the diaphragm. This atonia prevents us from moving around and hurting ourselves or bed partners during sleep, but it can also contribute to the etiology of certain sleep disorders, such as narcolepsy.
Musculoskeletal disorders of the bones, joints, and muscles — like arthritis, fibromyalgia, or chronic back pain — can result in pain that leads to microarousals with movement and interferes with the ability to sleep. This problem with sleep leads to fatigue, interferes with the person’s quality of life, and can make their pain worse56. Treating such pain and insomnia concurrently — rather than sequentially — yields better outcomes for both conditions, compared to maximizing treatment for one condition and subsequently adding treatment for the second.