Chronotype and bruxism: Should we look further and get it from the heart?

Dear Editor: The emergence of associations between chronotype and psychosocial stress have led to an emerging interest in the impact of circadian elements regulating internal timing on bruxism. The recent stimulating paper of Serra-Negra et al. [1] on the association between subjective reports of bruxism (SRB) and chronotype suggested that the eveningness chronotype appears to be a risk factor for awake bruxism (AB) in dental students older than 22 years of age. Although a potentially intriguing finding, results should be cautiously interpreted and may benefit from a more integrative perspective, as related to either the circadian physiological mechanisms or to the potential clinical impact of the eveningness in SRB. It is now well accepted that improved knowledge on the circadian aspects of physiology and behavior could facilitate a better understanding of the pathways and mechanisms underlying controversial aspects of this particular motor expression within the orofacial region. The circadian timing system (CTS), the neural factory of the so-called internal clock, is a very complex network providing accurate temporal definitions to virtually every cell of the body. Chronotype as a construct reflecting individual differences in diurnal preference is not an independent trait of this “time machine” [2]. Actually, as stated by Roenneberg, et al. [3], determinants and outcomes of chronotype result from an interplay between circadian physiology, genetics, and behaviors in the context of an individual’s social and solar clock. Sleep-related movement disorders, a major class of sleep disorders, exhibit important associations with cardiovascular disease (CVD) [4]. Bruxism, either awake (AB) or sleep-related bruxism, was suggested as an early sign of CVD, even in the absence of other comorbidities such as obstructive sleep apnea [25]. This hypothesis was derived in part from the observation that orofacial muscular activity and increases in heart rate were temporally concurrent in several experimental models [6]. However, attempts to corroborate the assumption about an interaction between bruxism and cardiometabolic dysfunction did not pan out, even though other sleeprelated movement disorders that exhibit such an association share common pathophysiological pathways and substrates with bruxism. Again, association and causality are two worlds in supporting evidence; caution is mandatory before interpreting findings from a small sample size group. Also, dental students undergo intense learning and training schedules that do not allow for wake and sleep adjustments, and these behaviorally determined altered patterns may hamper the interpretation of the findings. Additionally, the lack of assessments of other comorbidities, such as anxiety, stress, sleep bruxism, and insomnia, may jeopardize the real impact of such contributing factors to chronotype definition. However, such preliminary observations open interest to further assess the role of chronotype on the apparition or awareness of AB. Indeed, we also investigated AB chronotype distribution in dental students in a recent non-published analysis. In a total of 133 students from the Faculty of Medicine in Coimbra, Portugal, we observed a high prevalence of subjectively reported prevalence of bruxism (51.9%). Furthermore, we evaluated the distribution of SRB manifestations across the 24 h of the solar day and found the following differences: morning complaints (18.6%), eveningness complaints (22%), undefined/ without an established temporal pattern (13.6%), and sleeprelated complaints (16.9%). We also noted that 28.8% of the SRB cohort reported no symptoms; such a high rate of asymptomatic individuals suggests that some different phenotype and endotype may be present in students reporting bruxism. We further characterized the chronotype profiles of our participants, whereby the intermediate chronotype was present in 57.6%, with the eveningness type being found in 28.8% and the morningness type in only 13.6%. In addition, we were able to collect ambulatory blood pressure (ABP) in the morning, post-prandially, and in the afternoon over three different days. Interestingly, even though all participants were normotensive, there was a trend to an altered pattern (i.e, an increased rather than hypotensive physiological response) of ABP rhythms in awake bruxers compared to sleep bruxers, CRANIO®: THE JOURNAL OF CRANIOMANDIBULAR & SLEEP PRACTICE 2021, VOL. 39, NO. 5, 457–458 https://doi.org/10.1080/08869634.2021. 1956786