## 4 Discussion
By understanding the interplay between individual characteristics and physiological traits, it would be possible to directly associate physiological outcomes with self-rating scales for stress and to better interpret, in turn, the induced effects from VR interventions. Hence, the development and implementation of common protocols for VR applications could help researchers address this major issue. In the future, selecting and combining data elements from various investigations may increase the impact of the results or expand the population to which the study results can be generalized, and accelerate knowledge growth [85]. From the examined studies that evoked an effect either on perceived stress or on physiological parameters, VR was mainly used in two ways to promote stress management in healthy populations. The majority of the authors (68.75%) employed immersive natural landscapes or resting indoor environments with biophilic elements to explore the restorative effects of VR on participants' stress [31; 46; 48; 49; 50; 52; 55; 56; 57; 58; 59]. Generally, nature is considered to have a positive impact on human health. This is scientifically supported by previous research, which has pointed out the \"added benefit\" to mental health from natural environments and, also, their ability to promote recovery from stress [86; 87; 88]. This preference for green spaces can be thoroughly explained by well-known theories that focus on the importance of nature to restore individual's mental and physical strengths. [PERSON]'s stress reduction theory (SRT) [73; 89; 90] emphasizes the physical and emotional advantages of exposure to green spaces and their effect on mitigating stress reactions. In particular, a natural environment with dense vegetation or water elements, which is perceived as unthreatening and safe, can activate a positive emotional reaction engaging processes that facilitate wakefully relaxed attention, and reduced autonomic and somatic responses. Studies on natural scenes have shown, in fact, that stressed individuals can reduce their tension via the observation of natural elements, which also involves lower levels of negative affect and quicker physical recovery from stress [34; 89; 90]. Similarly, [PERSON]'s attention restoration theory (ART) [89] is concerned with natural spaces and proposes that nature enables the restoration of attention capacities, which would otherwise be depleted by demanding, effortful activities. According to ART, natural environments owe their regenerative effect to four main qualities: (a) they are _fascinating_ and attract involuntary attention without exploiting cognitive resources, (b) they offer a feeling of _being away_, (c) they give the impression of _extent and variety_, and (d) they are _compatible_ with human existence. It seems that such preferences have their origins in evolutionary adaptive behaviors that were earlier essential for ensuring individual's survival and well-being. While immersive natural scenes appear to be frequently used in VR interventions, this fact does signal to future scientists that this area is a valuable starting point for proving further VR interventions' effectiveness on stress management. Generally, integrating natural elements into VR systems or combining them with other activities may be a promising tool for facilitating positive changes in subjective stress level and stress-related biomarkers. In the present review, the combination of VR with heart rate variability biofeedback (HRV-BF) was also interesting. In their study, [PERSON] et al. [47] conducted a single session of HRV-BF for a short duration, where the participants were requested to practice a slow diaphragmatic breathwork guided by auditory instructions. In the VR biofeedback condition, the individuals also experienced a virtual beach scenery. The authors argued that the use of standard HRV-BF demands the involvement of self-regulatory processes and increased attentional resources, which may add to the complexity of the task and may cause users' demotivation to continue the training.