Adolescent suicide has emerged as a critical global public health concern. The World Health Organization (WHO) reports it as the third leading cause of death among individuals aged 15-29 years. During this developmental stage, individuals experience not only rapid physical growth but also critical psychological and emotional maturation. Adolescents exhibit significant fluctuations in emotional and psychological states, which are strongly influenced by environmental and social factors, rendering mental health issues during this period exceedingly complex. This complexity is particularly evident among individuals with depression, where suicidal ideation (SI) is more prevalent and severe. Research indicates that individuals with depression experience substantial short-term emotional fluctuations, and this instability may be closely linked to the development of SI (Bowen et al., 2015; Peters et al., 2016). However, the nuanced mechanisms underlying this association—particularly across diverse cultural and social contexts—remain understudied.

According to the International Classification of Diseases, 11th Edition (ICD-11), the diagnostic criteria for major depressive disorder primarily include persistent low mood, which typically presents as relatively stable for most of the patient's time (Bos et al., 2019; Koval et al., 2013). However, recent studies indicate that individuals with depression exhibit significantly higher emotional volatility in daily life, particularly in the short term, compared to the general population (Bos et al., 2019; Judd et al., 2013; Köhling et al., 2016). Emotional volatility encompasses not only fluctuations in negative emotions but may also involve the absence or inconsistency of positive emotions (Raudales et al., 2021). These fluctuations are closely related to an individual's SI; the recurrence of suicidal thoughts can lead to an increased number of suicide attempts, ultimately resulting in completed suicide (Park et al., 2020). The relationship between emotional states (including both positive and negative affects) and SI is especially complex in adolescent populations. Existing literature widely suggests that positive emotions can alleviate psychological stress to some extent and enhance psychological resilience, thereby reducing suicide risk (Carl et al., 2013). Fredrickson's "Broaden-and-Build Theory of Positive Emotions" posits that positive emotions expand individuals' cognitive and behavioral repertoires, subsequently strengthening psychological adaptability (Fredrickson, 2001). Conversely, negative emotions, such as anxiety and depression, may increase the incidence of SI by amplifying emotional distress and negative cognitions (Chen et al., 2020; Tello et al., 2020). Additionally, emotional fluctuations during adolescence are influenced by a complex array of social and environmental factors; negative emotions can trigger detrimental cognitions, such as self-deprecation and hopelessness, thereby elevating the risk of SI.

In this context, daily emotional stability—defined as an individual's ability to maintain relative emotional steadiness when facing stress and challenges—has been identified as a crucial factor affecting SI. Daily emotional stability is considered a multidimensional psychological construct encompassing the perception of emotional fluctuations, emotion regulation abilities, and recovery from negative emotions. Individuals with poor daily emotional stability may exhibit greater vulnerability during emotional fluctuations, consequently increasing the risk of SI (Hallensleben et al., 2019; Kivelä et al., 2024). However, findings regarding the relationship between daily emotional stability and SI have been inconsistent. Some studies report a protective effect of emotional stability against SI (Baryshnikov et al., 2024; O'Connor and Portzky, 2018), while others find no significant association (Mallorquí-Bagué et al., 2018; Pisetsky et al., 2016), or even note that extreme stability in negative affect may reflect emotional blunting—a risk factor for suicide (Auerbach et al., 2020; Grigoriou and Upthegrove, 2020; Krynicki et al., 2018). These discrepancies may stem from methodological differences (e.g., retrospective vs. real-time assessment), variations in sample characteristics (e.g., clinical severity, presence of personality disorders), or cultural moderators influencing emotion regulation norms and the expression of distress. Thus, it is necessary to adopt novel research methodologies to explore the specific mechanisms by which daily emotional stability influences the development of SI in adolescents.

In recent years, the application of Ecological Momentary Assessment (EMA) in mental health research has provided new methodological approaches for understanding the relationship between emotional volatility and SI. EMA captures dynamic changes in emotional and psychological states through multiple real-time measurements of individuals in their natural environments, effectively avoiding the recall bias inherent in traditional questionnaires (Burke et al., 2017; Davidson et al., 2017). EMA holds significant advantages in exploring short-term emotional fluctuations and their effects on suicide risk, enabling the capture of real-time changes in emotional states and their direct impact on behavior (Oquendo et al., 2021; Peters et al., 2022). Therefore, this study aims to utilize EMA technology to capture the dynamic fluctuations of emotions and SI in adolescents with depression at a higher temporal resolution.

This longitudinal study aimed to explore the dynamic relationship between emotional instability and suicidal ideation (SI) in adolescents with depression, utilizing Ecological Momentary Assessment (EMA).Our specific objectives were to: (1) examine the association between instability in both positive and negative emotions and fluctuations in SI; (2) analyze the role of specific emotional subtypes in different dimensions of SI (desire, intention, resistance ability); (3) compare the predictive value of emotional instability derived from traditional retrospective questionnaires versus real-time EMA measurements; and (4) investigate whether emotional instability during hospitalization influences short-term treatment response and symptom relapse.By capturing real-time, high-frequency data on emotional states and SI, this EMA-based design offers enhanced ecological validity and minimizes recall bias, representing a key methodological advancement.We employed elastic net regression and linear mixed-effects modeling to robustly analyze these complex, nested temporal data.The findings are expected to address significant gaps in the literature regarding the moment-to-moment mechanisms of suicide risk, providing a stronger empirical foundation for developing targeted, real-time intervention strategies for this vulnerable population.