BACKGROUND: Antidepressants are increasingly used in children for various psychiatric disorders but also for sleep disorders such as insomnia; however, it is currently unknown how many children undergoing polysomnography (PSG) are taking anti-depressants. The aims were: to determine the frequency of use of antidepressants in paediatric patients referred for PSG, to identify the most common antidepressants used, to investigate the reasons for their use, and to analyse the PSG parameters found in children taking antidepressants. METHOD: An observational cross-sectional retrospective chart review of all children undergoing PSG at Seattle Childrens Hospital from 6/14/2020 to 12/8/2022 was carried out. Clinical features (such as diagnosis, especially psychiatric), sleep disorders (such as insomnia and restless sleep), and class of antidepressant used [selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCA), or atypical antidepressants], and PSG parameters were collected for further analysis. RESULTS: Among 3,371 patients who underwent PSG during the study, 367 children were selected who were taking one antidepressant only (154 boys and 213 girls, mean age was 13.7 ± years 3.69). A significantly decreased sleep stage N3 was found in girls, who were older than boys. Children with insomnia had longer sleep latency than children without, but more N3. There was a prolonged rapid eye movement (REM) sleep latency in children with attention-deficit/hyperactivity disorder and children with autism. REM latency was longer and REM percentage smaller in children taking SNRIs. Periodic leg movement index ≥ 5/hour was found in a higher number of children taking SSRIs or SNRIs (24.9%) than in subjects taking TCA or atypical antidepressants (13.3%) (chi-square 5.29, p = 0.013). CONCLUSIONS: Child and adolescent psychiatrists should question about the effects on sleep (both positive and negative) after initiating therapy with antidepressant medications.