## 4 Discussion  
Several history reports and clinical signs and symptoms have been suggested as markers of SB (e.g., report of sleep teeth grinding sounds, shiny spots on restorations, masseter muscles hypertrophy, transient jaw muscle pain in the morning, tooth wear etc.) although none of these has been directly and consistently associated with ongoing SB [6]. However, the presence of at least one of the above clinical symptoms or signs, together with the bed partner's report of tooth grinding during sleep, has been used to propose screening criteria for PSG-based SB assessment [5; 15]. These criteria were very useful in the research setting, but their validity for recognizing clinically relevant SB has been questioned. In particular, the peculiar study populations recruited for research purposes and the focus on the number of SB events, instead of the full spectrum of sMMA activities, emerged as shortcomings to address for further refining of the SB epidemiology [13]. Within this premise, the present investigation provided information on the frequency of sleep bruxism behaviors by EMG/ECG recordings with a validated portable device [10; 12], with the aim of collecting data on the frequency and natural variability of SB in healthy individuals. Results show that in a population of healthy young adults, the average SB index over four consecutive recording nights was 3.6 \(\pm\) 1.2. The majority of sMMA were tonic (49.9%) and phasic (44.1%), whilst mixed sMMA events (6%) were less frequent, in accordance with a previous study [17]. As regards gender comparison, no significant differences were found in any outcomes except for mixed sMMA events. These results are partially in accordance with the literature on self-reported bruxism, which did not report any statistically significant gender differences [8; 18]. However, non-instrumental findings of a slightly higher report of SB in females are worthy of further examination. As for SB frequency fluctuation, no statistically significant differences for any study variables during the four nights were detected. This suggests the absence of a first night effect (FNE), which is likely due to the smaller size of the device with respect to that used in another recent investigation reporting a potential FNE in the evaluation of SB [19] or to the peculiar sample of healthy individuals with respect to another study reporting a possible FNE in subjects with SB [20]. Our findings are open to interesting observations both for research and clinical purposes. The average values for the SB index over the four recording nights (i.e., 3.6 events/h) are very similar to the cutoff threshold that has always been accepted to mark high frequency SB (i.e., 4 events/h). Our results are in line with the previous investigations, which reported average SB index values of 4.5 \(\pm\) 2.6 [21] and 3.3 \(\pm\) 1.7 [17] events/h in a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Male**} & \multicolumn{2}{c}{**Female**} & \(t\)**-Test** \\ \cline{2-6}
**Outcome** & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**Range**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**Range**} & \\
**Variable** & \multicolumn{1}{c}{**(SD)**} & \multicolumn{1}{c}{**(Min-Max)**} & \multicolumn{1}{c}{**(SD)**} & \multicolumn{1}{c}{**(Min-Max)**} & \\ \hline SB Index & 4.1 (2.1) & 0.0–9.0 & 3.3 (1.8) & 0.0–8.5 & 0.058 NS \\ Phasic sMMA & 1.9 (1.5) & 0.1–6.7 & 2.3 (1.6) & 0.1–6.7 & 0.213 NS \\ events per hour & & & & & \\ Tonic sMMA & 2.1 (2.1) & 0.1–10.5 & 2.7 (2.4) & 0.0–12.0 & 0.174 NS \\ events per hour & & & & & \\ Mixed sMMA & 0.2 (0.3) & 0.0–0.8 & 0.4 (0.4) & 0.0–1.2 & 0.024 S \\ events per hour & & & & & \\ \hline \hline \end{tabular} SD, standard deviation; SB, sleep bruxism; sMMA, sleep-time masseter muscle activities; S: significant (\(p<0.05\)); NS: not significant.  
\end{table}