Circadian rhythms, insomnia and osteoarthritis pain

In this outstanding article published in the recent issue of Chronobiology International, Spira et al. (2015) investigated the sleep–wake cycle of individuals with knee osteoarthritis (OA) using actigraphy. The authors highlighted the importance of specific research about the relationship of disturbed sleep (insomnia) and musculoskeletal disorders (OA) in the field of sleep and chronobiology. Several studies have been reported about the cyclic intricate relationship of sleep and pain that promote a marked strength in the direction of sleep disturbances generating or aggravating pain (Finan et al., 2013; Tang et al., 2012). The sleep–pain relation is often mediated by depressive symptoms, particularly when pain is severe (Parmelee et al., 2015). Sleep stages are differentially sensitive to pain (Andersen & Tufik, 2003; Carli, 1987) and persistent pain such as OA, influence sleep stages and its architecture. Studies about sleep architecture and OA, both in animal models and in patients demonstrate that sleep disturbances may contribute to unrefreshing sleep, pain and fatigue symptoms. Our group investigated the sleep architecture in an animal model of OA and found decreased sleep efficiency, slow-wave sleep, Rapid Eyes Movement (REM) sleep and fragmented sleep (Silva et al., 2008). Evidence shows that sleep pattern (architecture and microarchitecture of sleep) changes with OA pain (Pickering et al., 2015). Moreover, chronic pain also influences microarchitecture of sleep [e.g. alpha electroencephalographic (EEG) sleep, which is intrusion of alpha waves overriding the normal sleep EEG] (Mahowald & Mahowald, 2000; Moldofsky, 1986). Collectively, these investigations point to an intrinsic relationship between OA and sleep disturbances, indicating a complex nociceptive process. The article by Smolensky et al. (2015) pointed out to a number of medical conditions, including musculoskeletal pain, having sleeping disturbances as a common outcome. Thus, circadian rhythmicity of many common diseases with pain and sleep disruption as symptoms requires to be explored in 24 h patterns of acute and chronic medical conditions, in order to establish chronopreventive and chronotherapeutic strategies. Sleep–wake cycle is intrinsic in sleep pattern, as it is an active process, which interacts with circadian physiology. Moreover, in sleep, we must consider its temporal allocation and its rotation between rest– activity states (Enright, 1980). Beyond and in addition to sleep pattern, circadian homeostasis is an important variable to investigate in research on chronic musculoskeletal pain states and misalignment in circadian rhythms, which this investigation by Spira et al. (2015) begins to address. To corroborate with their findings, another recent study shows an association between chronic disruption of circadian rhythms and the development of OA-like pathology in an animal model (Kc et al., 2015). Specifically, they found that phase-shifting activates catabolic signaling and suppresses chondroprotective pathways in the animal model knee joint, identifying circadian rhythm disorders as a putative novel risk factor for OA (Kc et al., 2015).