Table 2.

Major findings, strengths, and limitations of the included studies on the bidirectionality of sleep and behavior in human subjects, ordered by study design.

Authors	Major Findings	Strengths	Limitations
S1: Conway et al. (2017) (Longitudinal)	One-way association, in boys but not girls, between externalizing behavior and increase in bedtime resistance was observed. In boys with high externalizing behaviors, bedtime resistance was observed to grow. Bidirectionality between trouble getting to sleep and externalizing behaviors was not significant.	Large sample size. Concentration on distinct sleep disturbances rather than general sleep difficulties.	Results might be non-generalizable to groups other than European Americans. Non-clinical population was assessed – described relationships between measured variables might be different for toddlers with sleep disorders. Reliance on mothers’ reports. No objective sleep measures.
S2: Steinsbekk & Wichstrøm (2015) (Longitudinal)	ADHD symptoms were unidirectionally linked with increased risk of insomnia. Insomnia and oppositional-defiant disorder were reciprocally related. Insomnia unidirectionally predicted conduct disorder.	Large sample size. Focus on distinct sleep disturbances and distinct mental disorders associated with them. Use of longitudinal data.	Results might be non-generalizable to other countries than Norway due to different frequencies of psychiatric disorders. Symptom counts were used instead of diagnoses of mental disorders for power reasons. Use of parent-reported measures.
S3: Williams et al. (2017) (Longitudinal)	Bidirectional link between self-regulation and sleep was observed. Authors discovered that the higher attentional regulation at 2–3 years old, the smaller the number of sleep difficulties two years later. Attentional dysregulation was indirectly linked to sleep-related issues: lower attentional regulation for 4–5 year-olds was related to emotional regulation issues at 6–7 years old which, in turn, was associated with higher levels of sleep behavioral problems two years afterwards. Behavioral sleep problems result in emotional regulation which, subsequently, affects attentional regulation and further impacts sleep problems.	Large representative sample size. Use of longitudinal data.	Use of maternal reported measures. Non-clinical population was assessed; described relationships between measured variables might be different for individuals with psychiatric diagnoses.
S4: Kouros & El-Sheikh (2015) (Longitudinal)	Children’s daily mood was discovered to act as a mediator between bad sleep quality and externalizing as well as internalizing problems. Bidirectional link between sleep problems and externalizing behaviors was present; daily negative moods may connect sleep with behavioral outcomes. Potentially bigger significance of sleep quality rather than sleep duration in the context of negative behavioral effects was observed.	Ethnically and socially diverse sample. Use of objective sleep measures.	Reliance on self-reported and parent-reported measures. Gained insights were based on correlational data which means that other variables unaccounted for by the authors could have contributed to the observed results.
S5: Mulraney et al. (2016) (Longitudinal)	Bidirectionality between sleep problems and externalizing behaviors was not seen in children with ADHD, while weak bidirectionality between sleep problems and internalizing behaviors was observed. State of sleep, behavior, and emotions was found to be consistent over one year.	Participants representing diverse cultural and social groups. Assessment of sleep and behavior through extensive methods with high validity.	Restricted variability of behavior and sleep difficulties levels in the participants. A period of one year may be too short to accurately gage bidirectionality of sleep and externalizing symptoms. Use of caregiver-reported measures. No data on sleep medication taken by children were gathered.
S6: Liu et al. (2021) (Longitudinal)	Sleep and behavioral problems were stable over time. Mother-reported sleep difficulties of their children at age 6 predicted subsequent externalizing and internalizing behavior reported by the youth at 11.5 years old (but did not predict the same variables gauged by mothers and teachers). Issues pertaining to externalizing, attention, and internalizing functioning assessed by teachers and mothers when children were 6 were associated with mother-rated and (or) self-rated sleep difficulties at 11.5 years old.	Large sample size. Implementation of valid and reliable measures. Use of assessments which included multiple raters.	No objective sleep measures. Lack of information about specific sleep disturbances as cause and effect of behavioral problems. Certain variables (demographic, psychological) were considerably different among participants that took part and didn’t take part in the follow-up.
S7: Quach et al. (2018) (Longitudinal)	In 6–7 year-olds, sleep problems predicted externalizing behaviors when they were 8–9 years old (and at no other age). Externalizing symptoms were a predictor for later sleep problems and internalizing symptoms, whereas internalizing problems were not predictive of future externalizing difficulties. The presence of intertwined sleep problems, externalizing problems, and internalizing symptoms was stable over time, suggesting that externalizing symptoms may act as a direct contributor to internalizing problems and an indirect contributor leading to them via sleep disturbances.	Large, representative sample. Implementation of invariance testing of the sleep problem factor.	Exclusively parent-reported measures were used. No objective sleep measures. Restricted ethnicity of participants.
S8: Williamson et al. (2021) (Longitudinal)	Bidirectionality between externalizing behaviors and behavior sleep problems was observed only in early childhood (and not in later childhood). Externalizing behaviors predicted behavioral sleep problems more strongly than vice versa.	Access to data from a long period of time, including crucial moments in children’s development. Large sample size.	Lack of participants suffering from chronic medical conditions; non-generalizable for this population. No objective sleep measures. Restricted ethnicity of participants.
S9: Kelly & El-Sheikh (2014) (Longitudinal)	Various aspects of sleep (sleep/wake problems, sleep duration, sleep quality) were predictors for long-term externalizing symptoms. On the other hand, although to a smaller extent, externalizing symptoms predicted later sleep problems which evidenced bidirectionality of this association. Sleep predicts and is predicted by adjustment of a child. Externalizing problems were associated with subsequent bigger sleep/wake problems, whereas depression and anxiety had different outcomes: shorter sleep duration as well as worse sleep quality and worse sleep respectively.	Implementation of objective sleep measures. Measuring different facets of sleep and their distinct psychological impact.	Use of self-reports. Different times of assessment (school year and summer vacations) could have an impact on sleep durations due to different sleep schedules characteristic for these times of the year.
S10: Bauducco et al. (2019) (Longitudinal)	Association between sleep quality/sleep quantity and impulsive behaviors were found out to be bidirectional. Reciprocal links were stronger for insomnia and impulsive behavior in comparison to sleep duration and impulsive behavior.	Large, community-based sample. Use of longitudinal data.	Conclusions based on one measure of impulsive behaviors. Reliance on self-reports for sleep quantity and quality. No objective sleep measures.
S11: Pieters et al. (2015) (Longitudinal)	Sleep disturbances were predictive of subsequent externalizing, internalizing, and substance use problems, whereas the converse relationship was not significant (apart from alcohol use).	Large sample size. Application of valid and reliable measures.	Use of only self-reported measures. Environmental factors were not examined.
S12: Wang et al. (2016) (Longitudinal)	The Troubled Sleepers group were discovered to have far higher levels of behavioral problems (attention, aggression) at age 17 in comparison to Normal Sleepers; at the same time, their emotional problems (anxiety, depression) were of a similar level. Bidirectionality of sleep disturbances and behavioral difficulties was observed, whereas emotional problems were unidirectionally predictive of later sleep problems.	Large sample size. Conclusions based on longitudinal data. Use of growth mixture modeling which made identification of various trajectories of sleep difficulties possible. Use of valid and reliable measures.	Non-generalizability to distinct sleep disorders caused by the assessment of general sleep problems. Use of self-reported and parent-reported measures. No objective sleep measures.
S13: Gregory & O’Connor (2002) (Longitudinal)	Early life sleep difficulties were predictive of attention issues and aggression 11 years later; however, the reverse is only true for attention problems predicting subsequent sleep disturbances.	Large sample size. Use of longitudinal data.	Sample was not ethnically diverse. No objective sleep measures. Exclusive reliance on parental reports.
S14: Shanahan et al. (2014) (Longitudinal)	Longitudinal bidirectionality between sleep problems and oppositional defiant disorder (ODD) was observed, with sleep problems being a stronger predictor for ODD symptoms than vice versa. When comorbidity was taken into account, conduct disorder and ADHD were not predicted by sleep difficulties anymore. Once comorbidity was controlled for, the majority of reciprocal links between sleep problems and mental disorders gathered around generalized anxiety disorder, depression, and ODD; this led to a hypothesis that irritability, a common denominator of these psychopathologies, might be a reason for that particular bidirectionality.	Assessment of the relationship between sleep and distinct types of mental disorder as well as their comorbidities.	Population from a restricted geographical area. Sample was not ethnically diverse. No objective sleep measures.
S15: Kortesoja et al. (2020) (Longitudinal)	Short sleep duration predicted subsequent emotional and behavioral problems more strongly than the converse. Larger behavioral and emotional problems were more predictive of sleep problems than vice versa. Self-reported sleep problems and externalizing behaviors (hyperactivity, conduct problems) were found to be bidirectional.	Large sample size. Conclusions based on longitudinal data.	Use of self-reported measures. No objective sleep measures.
S16: Vermeulen et al. (2021) (Longitudinal)	Bidirectionality between sleep length and externalizing problems might be attributed to environmental and genetic factors rather than the variables themselves. Reciprocal relationship between sleep problems and behavioral difficulties were observed, whereas sleep duration was not seen to be a contributing factor to maladaptive behavioral activity.	Large sample size. Use of longitudinal discordant MZ cotwin design.	Questionnaires with few items were used. No objective sleep measures.
S17: Wang et al. (2021) (Longitudinal)	A bidirectional relationship was detected between ADHD symptoms and sleep problems, with sleep being a stronger predictor of ADHD symptoms than vice versa. Sleep problems were found to be unidirectionally predicting conduct problems; the reverse relationship was non-significant.	Assessing the sleep-externalizing behavior relationship in a unique context (post-earthquake). Examining the relationship of sleep with different facets of externalizing behavior (ADHD symptoms and conduct problems).	No objective sleep measures. Data non-generalizable to the general population. Short period between collecting data from two waves.
S18: Kelly et al. (2022) (Longitudinal)	Bidirectionality between sleepiness and externalizing behavior was observed. Sleep duration was discovered to predict and be predicted by externalizing behavior.	Assessment of specific sleep variables (sleepiness, sleep duration). Implementation of both subjective and objective sleep measures.	Sleep stages, which may be an important factor in determining sleep-behavior associations, were not examined. Actigraphy was only employed during two weekend nights at each data collection wave.
S19: Van Dyk et al. (2016) (Cross-Sectional)	Increased externalizing symptoms were associated with shorter sleep. Every hour more in an individual’s sleep time led to a decline in mean externalizing problems. Unlike in the relationship with objective sleep duration, no significant bidirectionality between subjective sleep quality and externalizing problems was observed.	Psychological symptoms and sleep were gauged in the environment natural for the participants. Application of various multivariable assessment techniques. Assessing a clinical group of individuals, the study obtained sufficient power to elaborate on effects of the measured variables on a daily level which allowed more detailed insights into the dynamics of bidirectionality between sleep and externalizing behavior.	Small sample size. Bigger generalizations are not warranted. Possible changes in everyday youth’s functioning that could have influenced their sleep and mental health were not controlled for. No comparison group.
S20: Yaugher & Alexander (2015) (Cross-Sectional)	Bidirectionalities between impulsivity and sleep as well as between antisocial traits and sleep were found to be insignificant. Impulsivity was a predictor of subjectively reported poor sleep quality; however, it was not a predictor of objectively measured sleep duration and efficiency by actigraphy. Psychopathic (antisocial) traits TriPM scores were found not to be a significant predictor of subjective sleep quality. Similarly, they were not predictive of sleep duration.	Implementation of both subjective and objective measures to assess sleep. Assessment of the relationship between sleep and distinct types of psychological outcomes.	Overrepresentation of the Caucasian subjects (64.5%). Non-clinical population was assessed; described relationships between measured variables might be different for individuals with psychiatric diagnoses.