78 children with SDB participated in the SDB group. We have recruited them from those patients who visited the sleep laboratory of the hospitals. All of them were untreated before the study. Children with a history of developmental, psychiatric, or neurological disorders or who were on medication that known to affect sleep were excluded. After the physical examination, all of them underwent overnight polygraphy, which was performed using the Somnomedics SOMNOscreen plus device (Randersacker, Germany) at the Sleep Disorders Laboratory of Heim Pal National Pediatric Institute, Budapest, Hungary, and at the Department of Pediatrics, Clinical Center, University of Pécs, Hungary. SDB was diagnosed by a board-certified sleep physician. Patients who met the International Classification of Sleep Disorders criteria  for SDB were included in the study. Furthermore, we used OSA18 Questionnaire to measure the quality of life and to identify the daytime and nighttime symptoms of SDB . The average age of the SDB group was 6.7 years (SD) = 1.83) (minimum 4 to maximum 10 years; 32 females/46 males), of which 61 had OSA and 17 had primary snoring. The average Apnea-Hypopnea Index was 3.44 event/h (SD = 4.00) (minimum 0 to maximum 19.3 event/h), the average desaturation level was 87.37 % (SD = 6.91) (minimum 60 % to maximum 99 %), and the average body mass index was 17.13 kg/m2 (SD = 6.78). According to the previous studies, primary snoring has been linked to daytime functional and behavioral impairments [5,14, 25]. Therefore, we compared the performance of the SDB group to that of controls and did not examine the OSA and snoring subgroups separately.
The control group consists of 156 children (average age: 6.57 years (SD = 1.46), 80 females/76 males). The control group was recruited through collaboration with public schools. We matched the two groups on age (t(232) = 0.578, p = 0.564) and gender (c2(1) = 2.192, p = 0.139). They did not suffer from any developmental, psychiatric, or neurological disorders and were free of any sleeping disorders.
Based on the OSA18 questionnaire , we excluded those children from the control group who reported SDB like symptoms and reached at least 60 points or more on the questionnaire. The comparison between the SDB and control groups is presented in Table 1.
Attention Deficit Hyperactivity Disorder Rating Scale (ADHD-RS)
ADHD-RS is an 18-item rating scale designed to assess symptoms of inattention, impulsivity, and hyperactivity disorder. Each item is rated on a 4-point scale. The rating scale consists of two subscales based on the relevant symptoms: Inattention problems and Hyperactivity-Impulsivity . We used the parent rating scale version in the study.
Strengths and Difficulties Questionnaire (SDQ)
SDQ is a 25-item rating scale used to measure emotional and behavioral problems. Each item is rated on a 3-point scale. The SDQ has five subscales, four of them measure major difficulties commonly experienced by children (Hyperactivity, Emotional Symptoms, Behavioral Problems, Social Problems). One subscale to assess strengths (Prosocial Behavior). There is three versions of the scale: parent-report, teacher-report, and self-report versions. This study is based on the parent-report version of the scale .
Child Behavior Checklist (CBCL)
CBCL is designed to assess behavioral and emotional problems and competencies. The Hungarian version of the questionnaire consists of 46 items, each item is rated on a 3-point scale. The CBCL produces two factors: Internalizing, composed of subscales Anxious/Depressed, Somatic Complaints, Inattention Problems, and Social Problems; and Externalizing, consisting of Aggressive Behavior, Deviant/Rule-Breaking Behavior. The CBCL is available in three versions: parent report, teacher report, and self-report versions. We used the parent version of the scale .
OSA-18 is a caregiver-administrated quality of life survey that contains 18 items . The questionnaire is organized into five domains: sleep disorders, physical distress, emotional distress, diurnal problems, and caretaker preoccupation. Each domain can be scored on a 7-point Likert scale. The OSA-18 total score is the sum of all 18 items. According to Franco et al , a total score < 60, between 60 and 80, and above 80 suggests a small impact on the quality of life, moderate impact, and large impact, respectively.
The study was performed using the Somnomedics SOMNOscreen plus device (Somnomedics) according to the guideline. Polygraphic readings were evaluated by a physician experienced in sleep medicine. Breathing irregularities (apneas and hypopneas) were analyzed, and Apnea-Hypopnea Index (AHI) was presented as the number of apneas and hypopneas per sleeping hour. The desaturation index indicates the number of periods with desaturation (minimum of 3 % fall in oxygen saturation) per hour. Marcus et al.  defined the polysomnographic diagnostic criteria for childhood OSA based on that system, one episode of apnea during sleep is considered pathologic. The respiratory pause typically lasts for two or more breaths to be considered abnormal. In hypopnea, the respiratory amplitude is reduced by at least 30 % and occurs when the oxygen saturation drops by 3 %. We classified the severity of OSAS as mild, AHI = 1 event/hour with desaturation or AHI 2–5 event/hour; moderate, 5< AHI >10 event/hour; and severe AHI >10 event/hour .
All children with SDB underwent a standard pediatric overnight clinical polygraphy. Medical histories and physical examinations were conducted by a board-certified sleep physician before the diagnostic night. On the night of the study, the parents, most often the mothers, completed the demographic and behavioral questionnaires. Written informed consent was obtained from both parents, and procedures were verbally explained to the children before the commencement of the study. The participants did not receive any financial compensation for their participation. Ethics approval was obtained by the Ethics Committee at Heim Pal National Pediatric Institute, Budapest (number of the Ethical Approval: KUT-6/2017).
Statistical analysis was performed using SPSS 22.0 (Statistical Package for the Social Sciences). Gender proportions were compared between the groups using Pearson’s chi-square test. Group differences were tested with the Mann-Whitney U test. Spearman’s correlation was used to explore the relationship between respiratory variables and behavioral measurements.