Researchers frequently use scales that assess psychological states retrospectively (e.g., “Over the last two weeks, I felt...”) or as a trait (e.g., “In general, I feel...”), even though many psychological states fluctuate both over the course of a day and between days (e.g., ^{[1][2][3][4][5]}[1,2,3,4,5]). In turn, there is strong evidence suggesting that these dynamic fluctuations influence how individuals interpret and respond to ambiguous events in their daily lives ^[6][7][8][6,7,8]. For example, imagine a scenario in which all your co-workers were invited to a happy hour after work, but you were left off the invitation. If you are in a positive mood, you might perceive this as an oversight; however, if you are in a more negative mood, or in the aftermath of a night of poor sleep, you might perceive this as a purposeful exclusion or a deliberate attack. Because people experience ambiguous events every day—and given that subjective experiences and appraisals of events have been found to be a stronger predictor of psychological and biological outcomes than more objective assessments of life events ^[9][10][9,10]—it is important to better understand how fluctuations in important psychological states influence not just how individuals experience specific events, but also how these changes relate to biological and clinical states over time.

EMAs have been used in academic research since the 1990s, although their use has become far more common and effective as recent advances in technology increased accessibility. Prior to the adoption of this term and the development of its methodology, similar methods may have been used for momentary assessments in various fields, including for academic research purposes, police investigations, and journalistic reports. Today, EMAs generally take the form of brief surveys that are digitally sent to participants and which are intended to be completed in real time, as individuals are experiencing events in their daily lives. Participants can be assessed at random intervals or in conjunction with key events of interest. Oftentimes, EMAs assess people’s thoughts, feelings, or behaviors, although any variables of interest can be measured. Advantages of EMA survey methods include the reduction in recall bias and erroneous reporting due to forgetfulness, their ecological validity, and the use of repeated assessments over time, the latter of which enables investigations of complex, within-person, temporal interactions between processes such as mood, stressful life events, and health behaviors [11]. Further, the use of EMAs also removes concerns associated with between-person confounders, reduces concerns about time-varying confounders, and can reduce the downward bias on effect sizes induced by overly long assessment lags [12]. 

2. Psychological States

Well-Being

Well-being is associated with health-promoting outcomes, including better physical and mental health ^[13][17]. Well-being contains both a subjective component, which is often assessed with life satisfaction and affect measures, and a psychological component, which is often conceptualized as thriving and finding meaning in life ^[14][18]. Rather than being just the absence of negative factors, well-being is better conceptualized as the presence of positive factors. Well-being predicts health, happiness, and longevity ^[13][17]. Although well-being is somewhat more stable than other psychological states (e.g., stress levels) over time, well-being also varies across time and contexts in ways that are overlooked by traditional single-timepoint well-being measures ^[15][19]. In studies specifically assessing within-person fluctuations in well-being, well-being is positively associated with being in nature and engaging in physical activity and negatively associated with being at work (for a review, see ^[16][14]). However, well-being in EMA studies is typically assessed with a brief measure of positive affect or subjective happiness ^[16][17][14,20], as opposed to a more comprehensive measure of eudemonic well-being that assesses aspects of growth, thriving, or meaning.

Stress

Stress refers to a person’s subjective experience of being able to manage the tasks in their lives, and not to a specific stressful experience, which is called a stressor. Although a small to moderate amount of stress is generally considered to be normative, and even beneficial ^[18][21], high stress levels, especially over long periods of time, can negatively impact both physical and mental health ^[19][20][22,23]. Indeed, whereas acute stress is a very strong predictor of PTSD and depression, chronic stress dysregulates biological processes that cause or exacerbate a wide variety of conditions, including anxiety disorders, depression, psychosis, asthma, ulcers, diabetes, certain cancers, and autoimmune and neurodegenerative disorders ^[21][22][24,25]. In fact, nine of the top ten causes of death in the United States today are exacerbated or caused in part by stress ^[23][26]. Due to the high prevalence of chronic stress and its negative impact on health, many interventions have been designed to reduce stress levels and improve stress-related health outcomes (e.g., ^[24][25][26][27,28,29]). Assessing the success of these interventions is typically carried out using follow-up surveys administered 1–12 months following the completion of the intervention (e.g., ^[27][30]), in which participants indicate their stress levels over the last week or month. However, sparse assessments like these fail to capture the dynamic nature of stress, well-being, and health; moreover, they can be influenced by a person’s current psychological state in ways that can bias participants’ experiences over the last month. Infrequent assessments thus limit our understanding of how interventions impact people’s daily fluctuations in psychological states, preventing researchers from obtaining a mechanistic understanding of how interventions influence stress and well-being over time. Comparatively less research has investigated daily fluctuations in stress levels (vs. well-being) in non-clinical populations, thus limiting our understanding of how stress varies across time and in response to changing circumstances. However, evidence is emerging that stress levels are especially dynamic ^[2][28][29][2,31,32], with a large amount of variability day by day, as well as over the course of a day. Further, stress levels are affected both by the number and severity of stressors experienced, along with a wide range of psychological states. Resilience, or how well people manage stressors, is associated with less negative affect in response to stressors (as assessed by EMAs), specifically in individuals who have experienced early life stress ^[30][33].

Mood and Affect

Positivity of mood and affect measures are so strongly related to well-being that they are often used as a proxy for assessing well-being itself. In college students, for example, researchers have found that daily levels of positive affect are negatively associated with both daily levels of self-reported stress and the perceived stress scale and positively associated with flourishing, a measure of well-being ^[31][34]. Beyond just levels of positive affect, variability in affect is also associated with well-being. Ong and Ram [1] reviewed research on how affective variability, instability, inertia, and reactivity influence health outcomes and well-being, above and beyond general levels of positive affect. They posited that those with fragile high positive affect (i.e., positive affect that, while sometimes high, is also highly variable) might not experience the same good health as those with more stable high positive affect, although this can be missed in traditional research designs. Consistent with this hypothesis, research using EMAs has found that decreases in the positivity of mood (i.e., positive affective reactivity) following a stressful life event predicted an increased mortality risk about ten years later, whereas neither negative affective reactivity to stressors nor general levels of positive affect influenced mortality risk ^[32][35].

Social Safety

Humans are inherently social creatures, and our social interactions and their quality influence our stress levels and well-being by influencing perceptions of social safety ^[33][34][35][36,37,38]. Whereas social safety, connectedness, and inclusion predict positive life outcomes, loneliness, social isolation, and rejection predict negative health outcomes, reduced happiness, and reduced longevity ^{[36][37][38][39]}[39,40,41,42]. Research using EMA methods and sensing data have revealed that daily social interaction (both conversation frequency and duration) are associated with lower levels of perceived stress, more so than levels of social interaction averaged across days ^[31][34], highlighting the advantages of assessing daily levels of social interaction using EMA methods. In a group of older adults living with HIV, researchers found that despite social interaction being associated with higher pain and fatigue ratings later that day, participants were happier when they spent time with others compared to when they spent time alone ^[40][43]. Finally, during COVID-19 lockdowns, one study ^[41][45] used EMA methods to investigate the impact of face-to-face social interactions on mood and stress in real time and found that having at least one social interaction preceding an assessment predicted a more positive mood, more calmness, heightened energetic arousal, and reduced stress compared to assessments when no social interactions were reported. These within-person effects persisted even while controlling for peoples’ moods and stress levels reported in the prior assessment, indicating that social interaction, and not pre-existing psychological states, were driving these effects. Because social interactions have a large impact on stress, well-being, and health, they are also important to consider in studies assessing health-related outcomes. 

Energy and Arousal

Another state that varies considerably from hour to hour is energy and arousal levels. In children, researchers have found that high energy levels and low levels of tiredness over 30 min predict moderate to vigorous physical activity, which, in turn, predicts more positive affect, less negative affect, and increased energy ^[42][46]. One strength of using EMA methods to assess these associations is that researchers are able to understand the temporal relations between mental states and behaviors. In this study, for example, energy levels both predicted, and were predicted by, physical activity. Researchers have also used EMA methods to assess associations between exposure to childhood trauma and daily energy levels, finding that the more childhood trauma a person has experienced, the lower their momentary energy levels ^[43][47]. As shown here, the use of EMA methods can help improve our understanding of complex associations between psychological states and stress, well-being, and health by revealing the exact temporal relations between these processes, and by facilitating the discovery of psychological mechanisms that might mediate or moderate associations between things like childhood trauma exposure and mental health outcomes. Further, EMA methods can help investigators determine how interventions impact people’s daily psychological states. 

Present Focus and Mindfulness

Being focused on the present moment is generally associated with enhanced well-being ^[44][49]. Levels of present focus are positively associated with life satisfaction ^[45][50] and vary dynamically day by day in ways that predict daily fluctuations in well-being ^[46][51]. Mindfulness interventions are designed to increase present focus and have been found to be associated with reduced stress levels and greater acceptance and self-regulation ^[44][49], along with increased resilience, self-efficacy, and well-being ^[47][48][52,53]. Other mental health interventions have also been found to increase present focus. For example, participants who engaged in three different types of meditation-based mental training interventions (presence-, affect-, and perspective-focused interventions) all exhibited elevated levels of present focus in their daily lives after three months of training, along with an increased ability to cope with everyday stressors ^[49][48]. Using EMA methods to assess present focus may be especially useful in intervention studies, insofar as present focus might mediate the association between the intervention and positive outcomes in daily life. 

Burnout

Burnout is a state of chronic physical and emotional exhaustion resulting from prolonged exposure to stressors that is often characterized by feelings of depersonalization and reduced personal accomplishment ^[50][55]. Burnout has become a significant concern in the workplace, with some studies reporting that more than half of American workers currently experience at least moderate levels of burnout ^[51][56]. Indeed, the demands of fast-paced work environments, long working hours, and increasing expectations have led to a rise in burnout rates among employees, negatively impacting their overall well-being and job performance ^[52][57], including reduced productivity, higher absenteeism, and increased turnover rates, leading to economic losses and compromised work culture ^[53][58]. Burnout is not only detrimental to psychological health ^[54][59], but also has profound implications for physical health. For example, persistent stress and emotional exhaustion associated with burnout have been related to weakened immune system function ^[55][60] and an increased risk of cardiovascular diseases ^[56][61]. Prior research in this context has used EMAs to measure burnout by asking participants to identify the extent of their emotional, physical, and mental exhaustion, along with end-of-day job satisfaction and quitting intentions ^[57][13]. By collecting real-time data through mobile devices, EMAs provide a dynamic way to track an individual’s stress levels, mood fluctuations, and daily work experiences. This research has helped to identify patterns of stressors and triggers that contribute to burnout, allowing for more targeted interventions and personalized support. Beyond using EMAs to assess burnout, ecological momentary interventions (EMIs) may help prevent burnout before it occurs. EMIs, a term coined in 2005, are treatments provided to participants during their everyday life through a mobile device, either on their own or as a supplement to a different ongoing treatment ^[58][62]. EMIs intervene in one’s day and environment, encouraging a certain behavior or providing feedback in real time ^[59][63]. By delivering timely and context-specific support, EMIs provide employees with coping strategies, mindfulness exercises, and stress management techniques precisely when they are most needed. Such interventions can enhance self-awareness, foster resilience, and promote adaptive coping mechanisms ^[58][62] to reduce the risk of burnout and promote overall well-being in the workplace.