A Surgeon General’s report in the US is meant to make the population aware of an urgent public health issue. It is reserved for only those health challenges that are so concerning that they need immediate action. Of growing concern is how the current pandemic has further impacted the mental health of children and emerging adults, which was already at crisis levels before the pandemic [2]. Increasing evidence suggests that psychosocial factors associated with the pandemic, such as social distancing and increased loneliness, exacerbated mental health issues in youth worldwide, with depression and anxiety doubling in the United States since the pandemic began [3]. Even before the 2020 pandemic, the United States and many countries worldwide have been on the verge of a loneliness epidemic [4]. Since the pandemic began, not only has the number of people who report profound loneliness increased, but also the frequency of their loneliness, with lower age (i.e., emerging adulthood) as a risk factor ^[5][6][7][5,6,7].

Indeed, loneliness is now considered a serious public health concern worldwide ^[1][8][9][1,8,9]. The number of adolescents with elevated loneliness nearly doubled worldwide from 2012 to 2018, with girls seeing more significant increases [9]. Emerging adults globally have also shown increases in loneliness over the last 43 years [8]. The US often has some of the highest rates of loneliness. By region, adolescent loneliness increases were most prominent in English-speaking countries (e.g., UK, USA, and Canada) and orthodox countries, such as Russia, Bulgaria, and Central/South American countries [9]. Similarly, older adults in the US, England, Poland, and Spain report some of the highest levels of loneliness, ranging from 9% to 19% [10].

Loneliness is different from physical isolation, as people may feel lonely even in the presence of others [11]. Instead, loneliness is a distressing feeling that arises from dissatisfaction with social relationships—be it the quality, the quantity, or even a perceived discrepancy between the person’s desired interactions versus the reality [12]. Loneliness is so concerning because it is a severe health risk. Loneliness is associated with internalizing problems such as depression, anxiety, and suicidal ideation ^[13][14][15][13,14,15]. Additionally, loneliness is related to poorer sleep quality, less physical activity, increased cardiovascular risk, immune dysregulation, and even mortality ^{[16][17][18][19]}[16,17,18,19].

This study involved two phases examining the influence of social support and loneliness on mental health outcomes in US emerging adults during the pandemic. The second phase, a follow-up longitudinal wave, used cross-lagged analyses to investigate changes in emerging adults’ loneliness, social support, and mental health problems. 

Distinct from adolescence and young adulthood, emerging adulthood is a developmental period encompassing the late teens and the mid-to-late twenties, with a particular interest in ages 18 to 25 [20]. This is when individuals no longer feel like a child but do not believe they are adults (i.e., feeling “in-between”) [21]. Parents also do not view their emerging adult child as an adult yet [22]. This period is marked by many environmental changes and individual development, partly due to the rise of post-secondary education and a delay in commitments related to jobs and marriage [23]. During this developmental phase, individuals explore their identities more intensely, experience more instability, and focus more on themselves [21].

Because emerging adulthood is a period with a propensity for change and growth, it may put these children at greater risk for problems such as loneliness if these opportunities for change and growth are thwarted. Children in late adolescence and emerging adulthood often feel lonelier than individuals in other age groups, except those in very old age ^{[24][25][26][27]}[24,25,26,27]. Indeed, loneliness follows a U-shaped curve, peaking in both emerging and late adulthood ^[26][28][29][26,28,29]. Even more concerning is the finding that loneliness rates among this age group have increased steadily since the 1970s [8].

Emerging adulthood is also a developmental period during which individuals may be especially vulnerable to stressors. Most psychiatric disorders develop by age 24, with the onset ranging from around 15 years and peaking in the early 20s ^[30][31][30,31]. The onset of disorders during this time is especially alarming, considering how mental disorders may hinder the personal and professional growth of emerging adults [31]. The development of psychopathologies may be exacerbated by events specific to the time of life, as young adults often experience academic and professional stress about beginning college and making decisions about their careers [32].

The circumstances of the COVID-19 pandemic can change the trajectories of many emerging adults. First, social distancing limits their autonomy and freedom to explore their identity entirely. Social bonds are increasingly important during this time, and social distancing measures interrupt the development of these bonds, isolating emerging adults from family and friends. Longitudinal research of multiple generations showed that younger cohorts tend to experience more loneliness, indicating that the impact of social isolation and loneliness could be experienced more intensely by these emerging adults [33]. Indeed, emerging adults suffer from some of the highest rates of loneliness worldwide ^{[1][34][35][36][37]}[1,34,35,36,37].

Understanding loneliness in emerging adults is critical because it often exacerbates certain psychopathologies, including the ones examined in the current study ^[38][39][40][38,39,40]. Loneliness is distinct from depression and anxiety, and loneliness may be responsible for increasing these internalizing problems. Since the first outbreaks of COVID-19, psychosocial responses to the pandemic have been highly negative ^[41][42][41,42], persistent over time ^[42][43][44][42,43,44], and strongly associated with reported loneliness ^[45][46][47][45,46,47] and social isolation [48]. Reports from the early stages of the pandemic in mid-2020 suggest that emerging adults are at higher risk of loneliness ^[34][36][37][34,36,37] and experience disruptions to psychosocial health and well-being related to loneliness ^[15][46][49][15,46,49].

Though loneliness and depression are distinct constructs, they are related ^[38][39][38,39]. In a meta-analysis of 88 studies on loneliness and depression, Erzen and Cikrikci (2018) found that loneliness had an overall moderate effect on levels of depression [50]. The relationship between loneliness and depression can be seen in multiple populations; for example, Murata and colleagues (2021) found that loneliness predicted higher depressive symptoms for adolescents, adults, and healthcare workers during the pandemic [13]. In a three-year longitudinal study of adults aged 50 to 67, Cacioppo and colleagues (2006) found reciprocal influences between loneliness and depressive symptoms [51]. Similarly, early peer-related loneliness has been shown to predict higher depressive symptoms in adolescence [52] and middle and later adulthood [53].

Loneliness can also lead to symptoms associated with post-traumatic stress disorder (PTSD), even years after a trauma has occurred. For example, Shevlin and colleagues (2015) found that loneliness mediated the relationship between childhood trauma and adult PTSD [54]. More childhood trauma predicted higher levels of loneliness, which then predicted higher levels of PTSD in adults. According to the American Psychiatric Association (2013), symptoms associated with post-traumatic stress include experiencing sleep disturbances, negative mood alterations, emotional reactivity, and frightening dreams [55]. There is a well-documented trend that increases in subclinical PTSD symptoms or PTSS, which are associated with modern large-scale pandemics [56], such as SARS (2003) ^[57][58][57,58], influenza (2009) ^[59][60][59,60], Ebola virus (2014) ^[61][62][61,62], and MERS (2015) ^[63][64][63,64]. In a study of adolescents, adults, and healthcare workers during the 2020 COVID-19 pandemic, Murata and colleagues (2021) found that loneliness was the most common predictor across multiple psychological outcomes, including PTSS, in all sample groups [64]. Additionally, Kalaitzaki and colleagues (2022) found that higher levels of loneliness were associated with higher PTSS in Greece [65].

Loneliness also strongly correlates with physiological and reported stress [40]. Higher loneliness is associated with increased hypothalamic–pituitary–adrenal axis (HPA) activity (implicated in reactions to stress and the release of cortisol) and lowered immunity, subjective well-being, and executive functioning ^[40][66][40,66]. Thus, lonely people tend to experience higher rates of inflammation and morbidities ^[67][68][67,68] and have an increased mortality rate [69] compared to less lonely controls. In a longitudinal study of adolescents aged 15 to 20, Vanhalst and colleagues (2013) found that individuals who reported lower levels of loneliness throughout the five years of the study had the best psychosocial functioning, while those who experienced chronic loneliness exhibited the worst psychosocial functioning (e.g., higher perceived stress, depression, and anxiety) [70]. Contextualized with COVID-19, early studies demonstrated that levels of both perceived stress [71] and loneliness [46] reported by emerging adults during the pandemic significantly exceeded pre-pandemic levels.

The effects of loneliness are felt even while we sleep. Kurina and colleagues (2011) found loneliness to correspond with a direct increase in sleep fragmentation [72]. Similar studies examining sleep report links between loneliness and poorer sleep quality [73] and increased daytime dysfunction [74]. Conversely, individuals with higher support report better sleep quality [75]. More recently, in a propensity-score-matched case–control study in China, lonely people had longer sleep latencies, woke up more frequently at night, reported poorer subjective sleep, and were more fatigued during the day than their non-lonely peers [19]. Lonelier individuals experience less restorative sleep and often feel drained of energy, lacking the physical and emotional resources to cope with other stressors or any pre-existing health problems, with lonely individuals being 2.67 times more likely to sleep less than usual while also being 1.92 times more likely to sleep more than usual during the beginning of the pandemic ^{[19][75][76][77]}[19,75,76,77].

Furthermore, both energy level and sleep dysfunction have been separately correlated with loneliness ^[72][73][74][72,73,74], and those who experience social isolation tend to exhibit poorer sleep habits and insomnia [78]. Emerging adults who experience higher levels of loneliness are significantly more likely to report inferior sleep quality, reduced sleep efficiency, longer wake time before sleep onset, and more significant daytime dysfunction ^[79][80][79,80].

Social support is vital in reducing loneliness across all age groups and reducing vulnerability to adverse health effects associated with loneliness, such as stress and related health symptoms. It is crucial during significant developmental changes, as in emerging adulthood [81]. Generally, social support is suggested to reduce loneliness, improve quality of life, increase subjective well-being, and decrease depression and hopelessness in vulnerable groups, such as older adults ^[82][83][84][82,83,84]. Following traumatic events, research shows strengthened support from one’s social networks to moderate the relationship between anxiety and depression [85]. Social support also buffers against stress and its associated symptoms, such as depression, and improves overall health by reducing loneliness ^[86][87][86,87].

Though emerging adults were among the loneliest during the pandemic across age distributions ^{[34][35][36][37][88]}[34,35,36,37,88], increases in social support during COVID-19 have predicted lower levels of loneliness among samples of college students ^[89][90][89,90]. Reductions in face-to-face interactions due to social distancing protocols in 2020 and 2021 saw increased online interactions and social media usage in this group ^[91][92][91,92]. Interestingly, higher rates of online social interaction elicit perceptions of social support among emerging adults without physical connection [93] and mediate the relationship between the age group and loneliness [91].

During emerging adulthood, the nature of relationships also changes; most emerging adults report having close friends and romantic partners. These relationships are distinct and can provide different patterns of support ^[94][95][94,95]. For example, friends provided higher levels of companionship and intimacy, while parents provided more affection and instrumental help [96]. Both friend and family support have been linked to maintained or improved mental health during adjustments to college [97]. Friends, however, are often rated higher than parents for overall support [96], and the relationship between friend support and loneliness is more robust than it is for either family or significant other support ^[81][84][81,84].

Emerging adults spend increasingly more time with their friends and romantic partners; the positive qualities of both relationships are related to less loneliness [98]. However, these relationships are voluntary and can even be transient, given that emerging adults can enter and leave these relationships freely ^[99][100][99,100]. Even though emerging adults rely more on their friends, parental support is still essential to young adults. Indeed, emerging adults still view parental support as valuable [101].

Given the findings from previous research, this study will also examine whether loneliness mediates the associations between perceived social support and mental health. It was expected that perceived social support would influence feelings of loneliness. Loneliness, in turn, would influence the mental health of emerging adults. Loneliness has been found to mediate the link between social support and subjective well-being ^[102][103][102,103]. Latent growth curve mediation models using the Longitudinal Aging Study Amsterdam also found that social support was associated with being less lonely. In turn, decreased loneliness was related to enhanced cognitive performance [104]. During the early months of the pandemic, loneliness even mediated the relationship between social support and elevated hope in samples from the UK, USA, and Israel [105]. Finally, loneliness mediated the relationship between family support and depressive symptoms among emerging LGBQ adults who were living with their parents during the pandemic [106]. As such, we wanted to examine whether loneliness mediated the associations between social support and mental health during the first year of the pandemic.