Nowadays, nurses are facing unprecedented amounts of pressure not only due to the growing global health demands but also the current COVID-19 pandemic. The rapid spread of the virus during the last year has placed a huge burden on many health care systems scrambling to cover the needs for intensive care unit beds, personal protective equipment for both health care professionals and patients, and offer high-quality health services to their end-users (Arnetz et al., 2020). The current pandemic outbreak can be considered as a stressor that has significantly affected nurses' mental health. According to stress literature, when the existence of an organism is threatened by exposure to either a physiological or a psychological stressor, the system reacts to the stressful situation in a generalized, complex fashion (Matousek et al., 2010). The transactional model of stress postulates that one's perception of a stressor will depend on the degree to which an individual assesses this stressor as meaningful and relevant to them in a given context (Lazarus and Folkman, 1984). Further, meaningfulness will shape the strength of one's reaction to the event. On physiological level, stress triggers an initial activation of the sympathetic nervous and adrenomedullary systems resulting in increases in cardiac activity (Zhang et al., 2018). Furthermore, activation of the hypothalamus-pituitary-adrenal (HPA) axis provokes endocrine and immune changes leading to the release of cortisol and cytokines in an effort to re-establish body balance (Matousek et al., 2010). Stress research has showed that stressful events at the workplace can cause high physiological and psychological workload which can lead, in turn, to serious health problems and burnout [Lazarus and Folkman, 1984; Eatough et al., 2011; Velana and Rinkenauer (in press)].

In stressful occupations, such as nursing, the experience of stress deriving from lack of social support, heavy workload, conflicts with colleagues and other critical job-related factors is strongly linked with poor health (Tyler and Cushway, 1995). In fact, scientific evidence supports such an association, as stress in nurses can cause health problems and psychosomatic disorders, absenteeism, workplace injuries and errors related to patient care (Shirey, 2006). Moreover, study findings have indicated that nursing profession is associated with high rates of psychiatric outpatient referrals and suicide (Jones, 1987). More recently, in a cross-sectional study among 1257 Chinese health care providers from 34 national hospitals, female nurses, especially those who work in Wuhan, reported more severe symptoms on distress, anxiety and depression as compared to physicians (Lai et al., 2020). Another cross-sectional study that was conducted in Germany, indicated that nurses reported higher levels of stress and subjective burden as well as lower levels of job satisfaction and experienced support than physicians (Kramer et al., 2020). In a broader context, the present health crisis urges attention on nurses' mental health and on the strategies that should be developed to enhance their well-being and quality of life. Therefore, developing and implementing innovative approaches may be a best practice policy to reduce stress and improve health (Hatcher et al., 2006).

Improving resilience to stress and enhancing nurse's strategies to respond effectively to stressors are key issues facing many health care institutions during the COVID-19 pandemic. Resilience is considered one's ability to recover easily and quickly from adverse circumstances that happen over the course of their life (Zautra et al., 2010). Applying this notion to an organizational environment, resilience to stress implies that, in general, employees can respond in a productive way when encounter significant job-related changes or pressure to reach outcomes (Home and Orr, 1998). In nursing profession, this can be proved particularly challenging, as nurses often have to deal with human suffering, interpersonal difficulties and other job-related issues such as bullying and violence (Jackson et al., 2007), conditions that are associated with high levels of stress and demand adequate personal resources and coping strategies. Hence, enhancing nurses' resources and support may have the potential to develop their capacities to deal with stress and workplace adversities. The last years health care organizations around the globe have developed and implemented various individual-level interventions and strategies to empower employees to tackle setbacks at the workplace. In particular, interest has been growing in highlighting the effects of interventions on stress management and improving nurses' mental health, such as mindfulness, meditation and relaxation techniques (Delgado et al., 2017; Ghawadra et al., 2019). Furthermore, in his study, Cottrell (2001) showed that focused interventions for mental health nurses can enhance work-related factors, such as job satisfaction, and ameliorate stressors at workplace. Another literature review revealed that different stress management programs, such as training in therapeutic skills or in behavioral techniques, may help nurses address stress (Edwards and Burnard, 2003). Nevertheless, the rigor of a number of studies has induced methodological weaknesses related, for example, to measurement tools utilized, study sample size or statistical methods used for the analysis of their results. Although many interventions appear promising to effectively decrease stress and improve well-being among nurses, there is another body of evidence that indicates only moderate or no intervention effects, and calls for further research in this field (Chesak et al., 2019; Li et al., 2019). In light of the current contact restrictions, it still remains unclear what strategies would be suitable to tackle job-related stress throughout the COVID-19 crisis and the era after it.

Since nurses experience high levels of stress at the workplace, it is of vital importance to review and systematically evaluate the studies that utilized various individual-level strategies as a method to reduce their stress. Therefore, the present literature review aims to address the following issues: (a) to provide a thorough overview of the stress management interventions targeted at helping nurses develop skills to cope effectively with stress, (b) to identify measurement tools utilized to evaluate nurses' stress level (i.e., subjective and objectively measured parameters), and (c) to provide future research and practice with fruitful evidence-based directions adapted to the current restrictions.

This systematic review of the literature investigating individual-level interventions for stress management in nurses revealed a wide variety of programs that can be mainly classified into: (a) technology-based interventions for stress management and mental health either guided by a website or delivered through mobile phones, (b) mindfulness-based and spiritual interventions, (c) programs with cognitive-behavioral components, and (d) programs addressing body. In particular, there are some indications that technology-delivered interventions with relaxation components and stress management interventions comprising self-care skills, cognitive-behavioral components and relaxation might be effective in reducing stress among nurses and improving their mental well-being. In this direction, earlier research reviews have indicated that a wide range of interventions are available and seem promising for decreasing psychological distress (Delgado et al., 2017; Ghawadra et al., 2019; Bakker et al., 2020). Although evidence supports the effectiveness of these mechanisms in tackling stress, the rapid changes in health care systems and the unpreceded pressure that nurses experience, highlight the need to develop interventions adapted to the new overwhelming demands. Indeed, the prevalence of mental health problems among health care workers, especially during and after outbreaks, is high (Ricci-Cabello et al., 2020). Furthermore, these problems are usually associated with long-term mental health burden and thus, hinder the immediate response to health threats, such as the present COVID-19 crisis.

It is of utmost importance to develop and implement stress management interventions that are not only conveniently accessible in the workplace but also, they meet the strict conditions for minimizing human contacts. To this end, evidence-based interventions and self-care practices for those in immediate need delivered through digital technology seem to be a promising solution for combating the detrimental psychological and physiological impact among nurses. For example, there is recently an emerging body of studies that examines the implementation of a self-help virtual reality (VR) protocol to overcome the negative consequences of the quarantine by reliving stress among individuals (Riva and Riva, 2020; Riva et al., 2020). The protocol is designed to simulate a natural environment, while the user can perform daily exercises aiming at self-concertation and relaxation. By transferring this idea to a demanding workplace, where restrictions for social contact apply, it might be an effective way to enhance nurses' resilience and generally, improve their mental health. Therefore, future research will be needed to examine, if the continuous use of technology-based stress management and the refinement of its technological capabilities would lead to individually tailored self-help programs and to a more positive effect.

Furthermore, the most commonly utilized instrument for assessing the primary outcome of stress was self-reported scales. However, the current review of the literature identified some efforts from these interventions to include objectively measured parameters in order to explore the effect of strategies developed on physiological level. The experience of high job stress can cause alternations in the physiological processes that the human body mobilizes in an attempt to re-establish inner balance. In psychophysiological studies, the exposure to a stressful event is associated with intense cardiac activity (Johnston et al., 2016) which may be considered as a predisposing factor for the onset of lifestyle diseases, such as depression and heart disease. Additionally, the prolonged activation of the HPA axis can lead to increased concentrations of stress hormones harming the immune system (Aguilera, 2011; Herman et al., 2016).

The assessment of specific stress biomarkers could provide considerable advantages to future studies: such measures will allow researchers to define the outcomes of their interventions in a more systematic and precise way, taking into consideration the individual differences. It is well-known that people respond to potential stressors with great variability (McEwen, 1998). In this regard, diverse individual-related factors, such as gender, age, health status and personality characteristics, may regulate not only physiological reactivity to stress but also individuals' ability to activate their resources and cope with challenges. By capturing physiological responses, it would be possible to reveal which psychophysiological mechanisms are involved into resilience processes and how individual characteristics are interwoven in physiological traits. This may contribute, in turn, to a better understanding of human body and the implementation of effective stress management strategies based on objective indices. Moreover, concrete physiological outcomes can be directly associated with stress scales that may have implications for individual's health status, and can reduce confounding effects by response bias inherent in self-report ratings (Schnall et al., 1992; Bosma et al., 1997; McEwen, 1998). Therefore, future studies could utilize physiological parameters as indices for assessing their effectiveness to reduce stress and overcoming certain methodological issues of self-report.

There are some other issues that have been identified by the current literature review. The majority of the studies included have been designed and conducted in US, where the surrounding conditions and needs for mental health care among nurses may slightly differ from those exist around the globe. These scientific data and knowledge deriving from research on preventive programs may be usefully applied to the case of nurses in countries other than the US. However, the fact that different countries operate very different health care systems may imply that there are limitations in generalizing and integrating the study findings (Edwards and Burnard, 2003). In line with this, most authors of the identified interventions recognized the limited generalizability of their results. In fact, it was found that most of the interventions were developed for a clinical environment and in most cases, they reported a small sample size or a homogeneous study population. Nevertheless, no interventions were identified recruiting large random samples, for example, in nursing homes, elderly care or health care facilities for homeless, where nurses may have increased needs for help and be at higher risk for chronic stress and stress-related diseases. In general, research results should be generalized with caution and future studies may adjust their methods to the local conditions of health care professionals. Another issue related to generalizability of the study findings extracted is that, in many articles, p values were reported without including the effect size, which is a standardized measure to evaluate interventions' clinical utility. Therefore, this fact rendered the comparison of the results difficult and based on this one might question their generalizability to other settings. Although previous research on stress reduction in health care providers has also identified the same methodological problem (e.g., Edwards and Burnard, 2003; Bakker et al., 2020), it still concerns. Furthermore, future research may pay more attention into different nursing specialties by developing strategies to meet the demands of non-hospital-based institutions (Bakker et al., 2020). Another major limitation of the included studies was lack of long-term follow-up data. Although the majority of the studies effectively decreased work-related stress immediately after the intervention and highlighted the benefits of such interventions for enhancing nurse's mental health, only nine programs out of the 27 identified interventions indicated a long-term change in the measured outcome. Nurses might have shown temporary improvements in stress immediately following a stress management program but might have returned later to baseline levels, especially without continued support. Hence, future interventions are needed to include longer follow-up intervals that can more reliably indicate the extent of their effect.

Individual-level interventions and self-care strategies are core values for addressing the growing problem of stress among nurses. However, the question of what stress management programs would be effective to enhance nurses' personal resources to decrease stress throughout the pandemic still persists. This systematic review of the literature highlights the immediate need for evidence-based preventive interventions that may be delivered through digital technology combined with relaxation and cognitive-behavioral components to reduce stress and meet the current conditions that allow fewer human contacts. The integration of VR as a tool of stress management into mental health research has the potential to offer a radical transformation of the traditional intervention programs, allowing their users to meet the current restrictions for human contact. Therefore, special attention should be paid to advancing technology-based interventions that develop innovative self-help strategies, and to applying standardized objective measurement tools to allow the quantification of sensitive physiological indices and transferability of scientific knowledge. Further research is needed to develop preventive programs with long-term follow-up for those nurses who work in specialized care at non-hospital institutions so as to understand and meet their needs for mental health care. Nurses play an integral role in each health care system and should be provided with all these appropriate self-help strategies to enhance their resilience and create a healing environment, where they can better exhibit their skills and commitment to patients. Having healthy staff is essential to delivering high-quality health care and preventing serious mental health disorders at the workplace.

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

MV designed the study. MV, and GR performed the literature review, data extraction, and classification according to the pre-defined criteria. MV synthesized the study findings and drafted the manuscript. GR participated into the review process. Both authors contributed to the article, read and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.