## 4 Discussion  
The results obtained in this study confirmed the possibility of assessing vigilance degradations through the analysis of participants' head micromovements. A significant vigilance reduction at the end of the PVTs [57] was highlighted by participants' reaction times (RTs) and also confirmed by a previous study [58] (Figure 2). In this regard, participants' head micromovements were able to discriminate low- and high-vigilance conditions. In fact, when the vigilance decreased significantly, we found increased intensity and variability of micromovement acceleration over time (Figures 3 and 4). Meanwhile, the results derived from body postures showed that, although there was a positive trend for acceleration intensity, we did not find any significant differences between the high- and low-vigilance conditions, both for acceleration intensity and variability. Moreover, the analysis of head micromovement variability showed a positive and significant correlation with participants' RT (Figure 5), indicating that this parameter could be used to assess the current state of operators' vigilance while dealing with tasks. Taken together, the results hint at the possible implication of the vestibular system with the vigilance assessment. In particular, it seems that in a state of high vigilance, the cognitive resources are sufficiently balanced between PVT execution and the request of the head and the body to maintain their position, which is most likely in an apparent nonconscious way. Meanwhile, the low-vigilance condition leads to higher cognitive demand to hold the head steady, leading to an increase in conscious control of head micromovements, while body position is not affected. Lower resources are instead dedicated to PVT execution, leading to a decrease in performance, as highlighted by the increase in RT. As a matter of fact, previous studies have pointed out that, when the vestibular system is challenged, maintaining the same balance and posture starts to be cognitively demanding [1; 2; 66]. However, on the other hand, the incorporation of vestibular stimulation, like swinging on a swing during leisure time, as suggested in [67], over time could instead improve cognitive function. Although plausible, the link between vigilance and the vestibular system is far from being resolved by the present study. More focused studies should address the actual causal relationship between the two. The results of the present study also demonstrate that IMUs endow a cheap, direct, and reliable measure of participants' vigilance levels. Due to the integration of IMUs with neurophysiological devices, these kinds of data could also be combined for assessing vigilance changes in terms of participants' behavior and cognitive response. This integration could help to identify and, if necessary, alert operators of conditions of vigilance deterioration. There are plenty of operation environments in which it could be possible to integrate inertial measurements on already existing equipment, such as in aviation on traffic controller operator or pilots' headphones or in environments in which security helmets are used, for example, on construction sites for heavy machinery operators or  
Figure 5: Repeated measures scatterplots (subject data in different colors). (**A**) Correlation between Reaction Times and head micromovements intensity; (**B**) Correlation between Reaction Times and head micromovements variability. All correlations are significant with \(p<0.05\), highlighted in red in the figure. quality control. In this way, no additional device that could modify or impede a normal day-to-day work routine would be required, and the ergonomics of the existing ones would not be compromised, allowing researchers to overcome a possible barrier to the adoption of such devices. The outcome can be used to create a closed loop between the user and the machine and make them continuously interact to mitigate OOTL phenomena and improve both users' performance and task safety.