## 1 Introduction  
Bruxism is an oromandibular condition characterized by different activities of the jaw muscles (i.e., grinding or clenching of the teeth and/or thrusting or bracing of the mandible) and two distinct circadian manifestations, viz., awake bruxism (AB) and sleep bruxism (SB) [1]. Recently, a panel of experts proposed two distinct definitions for awake and for sleep bruxism; the latter is defined as a masticatory muscle activity during sleep that is characterized as rhythmic (phasic) or non-rhythmic (tonic) [1]. This would not be considered a disorder per se in otherwise healthy individuals, but it might be viewed as a protective and/or a risk factor for some clinical consequences [1; 2; 3].  
Whilst polisomnography (PSG) has always been considered the standard of reference to evaluate the amount of jaw muscle activity associated with specific patterns of sleep arousals, several different non-instrumental and instrumental approaches may be used to assess SB, including clinical or self-reported/questionnaire-based protocols, clinical inspection and electromyographic (EMG) recordings [1; 3; 4; 5; 6; 7]. In particular, a count of SB events can be assessed with measurement approaches to quantify the amount of sleep-time EMG activity [1].  
Current knowledge on the epidemiology of SB reflects the adoption of different evaluation strategies, since the literature reports varied ranges of prevalence for both children/adolescents and adults. Prevalence rates in adults range from 8% to 16% for sleep bruxism [3]. In 2013, a comprehensive review of the topic cautioned about the interpretation and generalization of findings due to the poor methodological quality of the reviewed literature, with special regard to the amount of papers relying on single-item self-reporting to \"diagnose\" bruxism [8]. A large-scale polysomnography (PSG)-based epidemiological study underlines that the prevalence of SB was 7.4% when PSG was used as an exclusive criterion for diagnosis, whereas it was 5.5% when screened by questionnaires and confirmed by PSG [9].  
As suggested by the consensus papers, an instrumental evaluation of muscle activity during sleep is the most appropriate approach to approximate a definite description of the epidemiology of SB behaviors [1]. Currently, a validated compact portable device has been proposed for the evaluation of SB, as an alternative to complete PSG, due to its high specificity and sensitivity [10; 11; 12].  
Based on this, also, given the progressive diffusion of portable EMG recording devices that may lead to an increased number of research in this field over the next few years, there is a need to provide a picture of the frequency and clinical relevance of SB. The first step is the assessment of the frequency and natural variability of different SB activities in healthy individuals [3; 13].  
Within the above premise, the twofold aim of the present study was to: 1. Assess the frequency and variability of sleep bruxism in a sample of healthy young adults who underwent home EMG/electrocardiographic (ECG) recordings with a portable device during four consecutive nights, and 2. Set a possible comparison standpoint for future studies.