Sleep is essential for overall wellbeing and is especially important for optimal development in the first 5 years of life (Bathory and Tomopoulos, 2017). Healthy sleep in toddlers is associated with positive cognitive, language, and motor development, as well as emotional regulation (Maski and Kothare, 2013; Morales-Muñoz et al., 2020). A common, empirically supported behavioral intervention to improve sleep is the institution of a consistent bedtime routine, indicated as five or more nights per week (Mindell et al., 2015, 2018; Meltzer et al., 2021). In addition to sleep benefits, bedtime routines are hypothesized to foster parent-child attachment, health behaviors, prosocial development, and social-emotional development (Mindell and Williamson, 2018). However, few studies have investigated the potential benefits of bedtime routines for early social-emotional development (Kelly et al., 2013; Zajicek-Farber et al., 2014; Ren and Hu, 2019), as well as its relationship with socioeconomic factors. In particular, lower caregiver educational attainment has been linked to less consistent bedtime routine implementation in prior research (Hale et al., 2009; Williamson and Mindell, 2020), but has not been examined in relation to both bedtime routines and social-emotional development. As the first 3 years of life is a sensitive period for early childhood development (Black et al., 2017), understanding variation in bedtime routine consistency, caregiver educational attainment, and their relation to toddlers' social-emotional development can help guide efforts to promote early childhood wellbeing.

General daily routines, including bedtime routines, are defined as a pattern of behaviors that are repeated consistently over time with active involvement of an adult (Fiese et al., 2002). From a behavioral perspective, a consistent routine acts as an environmental stimulus signaling the child to what is expected (Sytsma et al., 2001). Past research shows consistent routines are associated with decreased externalizing behaviors, as well as the development of better self-regulation and social skills (Bater and Jordan, 2017). Conversely, inconsistent routines or lack thereof make it difficult for toddlers to predict what activity comes next, which can yield unpredictable behavior such as tantrums, contributing to adverse child outcomes (Sytsma et al., 2001; Martin et al., 2012).

Based on the developmental theory that family routines positively impact child self-regulation (Sytsma et al., 2001; Martin et al., 2012), bedtime routines may be similarly beneficial. Bedtime routines are defined as a consistent set of caregiver-directed child activities, such as reading books, brushing teeth, and bathing, that occur every night in the hour before lights out (Mindell and Williamson, 2018). Furthermore, bedtime routine activities can be categorized into components, including nutrition, hygiene, communication, and physical contact, which can promote positive outcomes across developmental domains, including sleep, health, literacy, and attachment, among others. The benefits of implementing a consistent bedtime routine to child sleep are well-established in the literature. Past studies have found that implementing a consistent bedtime routine (i.e., five or more nights per week) is associated with an earlier bedtime, decreased sleep onset latency, decreased number and duration of night wakings, increased nighttime sleep duration, and fewer caregiver-reported sleep problems (Mindell et al., 2015, 2017, 2018; Fiese et al., 2021). A consistent bedtime routine also likely benefits young children's overall development. For example, institution of a language-based bedtime routine (e.g., parent-child book sharing, storytelling) at an early age is associated with better language ability (Hale et al., 2011), cognitive-academic skills (Câmara-Costa et al., 2021), and subsequent academic achievement (Câmara-Costa et al., 2021). Having a consistent bedtime routine is additionally associated with better executive functioning (i.e., inhibition-attention, working memory, and cognitive flexibility), school readiness, parenting style, and dental health (Kitsaras et al., 2018).

However, few studies have examined the potential benefits of bedtime routines on young children's social-emotional development. Social-emotional development refers to young children's ability to form and maintain meaningful relationships and express emotions. It encompasses prosocial behaviors (e.g., sharing and helping) and emotional and behavioral challenges (e.g., aggression and tantrums). One study of 2,977 children of low socioeconomic status (SES) backgrounds found that having a consistent bedtime routine at 36 months was associated with better concurrent emotional and behavioral regulation (Zajicek-Farber et al., 2014). Another longitudinal study found that irregular bedtimes were linked to child behavioral difficulties at ages 3, 5, and 7 (Kelly et al., 2013). Compared to those with consistent bedtimes, children with inconsistent and late bedtimes had higher scores on mother- and teacher-rated behavioral difficulties, such as hyperactivity, conduct problems, and peer problems. Moreover, children with irregular bedtime schedules at all three age points demonstrated a 3-fold increase in behavioral difficulties relative to those who only had one irregular bedtime schedule. In comparison, children who had an irregular schedule earlier in development followed by a consistent bedtime schedule by age 7 years old demonstrated improved emotional and behavioral outcomes. Although this study did not include bedtime routine implementation as a variable of interest, other studies have found that families who had irregular bedtime schedules were less likely to endorse consistent bedtime routines (Larsen and Jordan, 2020). Another study of school-aged Chinese children found that increased bedtime routine consistency was associated with better social skills, including communication, cooperation, and self-control, but not with problem behaviors (Ren and Hu, 2019). These studies all focused on preschool-aged and older children, with none involving toddlers.

Overall, much of previous research has primarily focused on the positive impact of a consistent bedtime routine on sleep outcomes. Yet, little is known about the relationship between bedtime routines and social-emotional development, especially during toddlerhood, a critical period of development. Furthermore, few studies have examined bedtime routine consistency in relation to developmental outcomes. Given well-documented pediatric sleep health disparities (Williamson et al., 2019, 2021; Williamson and Mindell, 2020; El-Sheikh et al., 2022), understanding these associations while also examining socioeconomic factors, in this case lower caregiver educational attainment, that may impact family resources to implement child bedtime routines (Hale et al., 2009; Williamson and Mindell, 2020) is crucial to inform equitable interventions.

Thus, the overall goal of the current study was to examine the longitudinal associations of bedtime routine consistency and caregiver educational attainment with social-emotional development in toddlers. We examined associations among bedtime routine consistency at 12 and 15 months with toddler social-emotional development at 15 and 24 months, covarying for concurrent bedtime routine consistency. We hypothesized that implementing a more consistent bedtime routine would be associated with later social-emotional competencies, fewer social-emotional problems, as well as fewer communication and personal-social concerns longitudinally. We also explored associations between caregiver education and toddler social-emotional development, and whether the associations among bedtime routine implementation and social-emotional outcomes varied according to caregiver educational attainment.

Overall, the majority of toddlers in this sample of families presenting to primary care had a consistent bedtime routine of five or more nights per week, between 63% at 12 months and 85% at 24 months. However, bedtime routine consistency was predictive of social-emotional outcomes. Toddlers with a consistent bedtime routine at 12 months of age had less behavioral dysregulation at 15 months. In addition, toddlers with a consistent bedtime routine at 15 months exhibited fewer externalizing and internalizing problems, as well as dysregulation, at 24 months, even after accounting for concurrent bedtime routine consistency. Bedtime routine consistency at 12 and 15 months was not associated with toddlers' communication and personal-social skills across time points. Lower caregiver educational attainment at 12 months of age was associated with later internalizing problems and dysregulation at 15 and 24 months, as well as maladaptive behaviors and lower communication skills at 24 months. Of note, there was a significant interaction between bedtime routine consistency and caregivers' education levels for future internalizing problems, such that toddlers whose caregivers had a high school education or less and lacked a consistent bedtime routine at 15 months displayed more internalizing problems at 24 months.

Our finding that toddlers with a consistent bedtime routine at 12 and 15 months exhibited less longitudinal behavioral dysregulation at 15 and 24 months and fewer externalizing and internalizing problems at 24 months aligns with previous findings in older children. For example, more consistent bedtime routines were linked with fewer externalizing behaviors and less bedtime resistance in a study of 120 predominantly White preschool-aged children (Larsen and Jordan, 2020). Another study of 87 predominantly Black/African American elementary school children from lower-SES backgrounds found that child-reported family routine (e.g., bedtime routine) moderated the relationship between teacher-reported child hyperactivity/impulsivity and oppositional defiant disorder symptoms (Lanza and Drabick, 2011). Additionally, in a population-based birth cohort study of 3,136 children in the Netherlands, Rijlaarsdam et al. (2016) also found that family regularity, including a consistent bedtime routine, at 2–4 years was longitudinally associated with a lower risk for child aggression in boys at age 6.

However, another study, using the same sample as Rijlaarsdam et al. (2016), found no longitudinal associations between family regularity at ages 2 and 4 and children's externalizing problems at age 6 (Hogye et al., 2022). It is possible that these studies yielded conflicting results because the latter study measured externalizing behaviors via child-report whereas other studies used caregiver-report to measure child externalizing problems. Collectively, however, most available studies suggest that a consistent bedtime routine is longitudinally associated with fewer social-emotional problems in older children. In our study, this relationship to outcomes at 2 years of age was stronger for bedtime routine consistency at 15 months versus 12 months.

Two underlying mechanisms may explain observed linkages between bedtime routine consistency and positive social-emotional development: (1) the regularity and predictive nature of a bedtime routine and (2) positive parent-child interactions during the routine. Presence of a consistent bedtime routine may act as an environmental stimulus to signal expectations to young children (Sytsma et al., 2001). In contrast, the absence of a bedtime routine or lack thereof, which may suggest a more chaotic household, could hinder toddlers' social-emotional development due to limited structure and predictability in daily activities (Bobbitt and Gershoff, 2016). Another possibility is that a bedtime routine generates rich opportunities for positive parent-child interactions, thereby fostering healthy social-emotional development (Mindell and Williamson, 2018). At bedtime, caregivers may have more opportunity to engage their child in interactive and stimulating activities, such as book-sharing and reading. Thus, caregiver involvement in a bedtime routine could include sensitive and responsive caregiving that cultivates secure child attachment (Bater and Jordan, 2017), thus facilitating healthy social-emotional development.

Contrary to our hypothesis, there were no longitudinal associations between bedtime routine consistency and toddlers' communication and personal-social skills across time points. These null results contrast with previous studies that found positive associations between bedtime routine consistency and these aspects of development. For example, a study of 228 school-aged Chinese children found that a consistent bedtime routine was associated with better social skills, including communication, cooperation, assertion, responsibility, empathy, engagement, and self-control (Ren and Hu, 2019). Other studies have found that consistent family routines, including a bedtime routine, are associated with better social skills in preschoolers (Koblinsky et al., 2006; Muñiz et al., 2014; Ferretti and Bub, 2017), whereas increased family chaos and less consistent routines are linked to decreased child prosocial behaviors (Bobbitt and Gershoff, 2016). Differences in child age could contribute to these discrepant findings, as these prior studies examined preschool-aged and older children, while our study focused on toddlers. It is also important to note that our small sample size might have yielded limited power to detect statistically significant associations with communication and personal-social skills. Furthermore, the ASQ-3 is a measure that focuses on developmental milestones and thus may not be as sensitive to more subtle differences in development, with modest accuracy in identifying early developmental delays (Sheldrick et al., 2020).

There are several limitations in the current study. First, the complete sample did not participate at all time points (70% at 15-months, 54% at 24 months). Although there were no significant sociodemographic differences between caregivers who completed the 15- and 24-month surveys and those who did not, as well as no differences in study participation rate of those with our without a consistent bedtime routine at baseline, our small sample size likely yielded limited power to detect statistical significance. This control sample was also drawn from a larger randomized trial, contributing to low power in this study. In addition, we relied on caregivers' report of their child's social-emotional development, which may be subject to bias and shared method variance. Future research may benefit from adopting a multi-informant approach (e.g., other family members and caretakers) and objective measures (e.g., behavioral observations) to assess both bedtime routines and social-emotional development.

The social-emotional measures were only administered at the 15- and 24-month visits, which limited our ability to control for toddlers' baseline social-emotional functioning and detect changes in functioning over time. Future research should administer social-emotional measures at baseline to better elucidate the difference in toddlers' social-emotional functioning at subsequent time points. Moreover, caregivers were not compensated for their participation. Although the primary care office administers many of these questionnaires as part of their routine visits, the lack of compensation may have contributed to the relatively high attrition rate. Also, generalizability of this study may be limited by the single primary care site from which families were recruited.

Additionally, although positive parent-child interactions during bedtime routines are postulated as an underlying mechanism to foster social-emotional development, we did not measure the quality of these interactions during the bedtime routine. Future research should examine these interactions to better understand their role in social-emotional development. It is also worth noting that institution of a consistent bedtime routine and social-emotional functioning may be bidirectionally linked (Williams et al., 2017; Quach et al., 2018). Thus, future studies should examine the reciprocal longitudinal interplay between bedtime routines and social-emotional development. Lastly, the current study did not measure caregivers' mental health (e.g., depression, anxiety) as a potential confound that might impede caregivers' ability to implement a consistent bedtime routine and might also negatively impact toddlers' social-emotional functioning. Future research should consider including caregivers' mental health as a contributor to these outcomes.

This study is among the first to examine associations among bedtime routines, social-emotional development, and caregiver educational attainment in toddlers. Toddlers with a consistent bedtime routine early in development, at 12 and 15 months of age, exhibited fewer social-emotional problems longitudinally. We also found that toddlers whose caregivers had a high school education or less and lacked a consistent bedtime routine at 15 months showed significantly more 24-month internalizing problems, even after controlling for concurrent bedtime routine consistency. These findings highlight the promise of a consistent bedtime routine as an intervention to promote toddlers' social-emotional development, beyond established sleep benefits. Bedtime routine consistency may be especially beneficial for toddlers with caregivers of lower educational attainment, although this finding should be examined in future research with a larger sample.

Nonetheless, given the critical growth and development that occurs in the first few years of life (Black et al., 2017), clinicians should consider recommending caregivers incorporate a simple, adaptive bedtime routine during well-child visits. A bedtime routine is a feasible and cost-effective behavioral intervention that could help families optimize healthy social-emotional development in toddlers. Institution of a consistent bedtime routine may also buffer against the early onset of social-emotional problems, including among toddlers of caregivers with lower educational attainment.

The datasets presented in this article are not readily available because the datasets generated and/or analyzed during the current study are not available for use outside of Saint Joseph's University and Temple University at this time, due to the nature of the ethics board approvals and possible risk(s) to study participants as well as the confidentiality promised to them. Data may be made available from the corresponding author on reasonable request with permission of study investigators and ethics board approval. Requests to access the datasets should be directed to jmindell@sju.edu.

The studies involving human participants were reviewed and approved by Lewis Katz School of Medicine, Temple University, Philadelphia, PA, USA and Saint Joseph's University, Philadelphia, PA, USA. Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin.

JM, AW, and MH contributed to the conceptualization and design of the study. JL, ZS, and MH participated in data collection. JL and ZS organized the database. JL, JM, and AW contributed to the statistical analysis. JL wrote the first draft of the manuscript. JM and AW wrote sections of the manuscript and contributed to manuscript revision. All authors have read and approved the submitted version.