Brigitte Holzinger,1,2 Franziska Nierwetberg,1 Frances Chung,3 Courtney J Bolstad,4 Bjørn Bjorvatn,5 Ngan Yin Chan,6 Yves Dauvilliers,7 Colin A Espie,8 Fang Han,9 Yuichi Inoue,10 Damien Leger,11 Tainá Macêdo,12 Kentaro Matsui,13,14 Ilona Merikanto,15,16 Charles M Morin,17 Sérgio A Mota-Rolim,18 Markku Partinen,19 Giuseppe Plazzi,20,21 Thomas Penzel,22 Mariusz Sieminski,23 Yun Kwok Wing,6 Serena Scarpelli,24 Michael R Nadorff,4,25 Luigi De Gennaro24,26 1Institute for Consciousness and Dream Research, Vienna, Austria; 2Medical University Vienna, Postgraduate Master ULG Sleep Coaching, Vienna, Austria; 3Department of Anesthesiology and Pain Medicine, University Health Network, University of Toronto, Toronto, ON, Canada; 4Mississippi State University, Mississippi State, MS, USA; 5Department of Global Public Health and Primary Care, University of Bergen and Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Bergen, Norway; 6Li Chiu Kong Family Sleep Assessment Unit, Department of Psychiatry, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, Hong Kong, Special Administrative Region, People’s Republic of China; 7Sleep-Wake Disorders Unit, Department of Neurology, Gui-de-Chauliac Hospital, CHU Montpellier, INM, Univ Montpellier, INSERM, Montpellier, France; 8Sir Jules Thorn Sleep & Circadian Neuroscience Institute (SCNi), University of Oxford, Oxford, UK; 9Department of Pulmonary and Critical Care Medicine, Peking University People’s Hospital, Beijing, People’s Republic of China; 10Tokyo Medical University, Tokyo, Japan; 11Université de Paris, APHP, Hôtel-Dieu, Centre du Sommeil et de la Vigilance, Paris, France; 12Department of Psychology, Federal University of Rio Grande do Norte, Natal, Brazil; 13Department of Laboratory Medicine, National Center Hospital, National Center of Neurology and Psychiatry, Kodaira, Japan; 14Department of Psychiatry, Tokyo Women’s Medical University, Tokyo, Japan; 15SleepWell Research Program, Faculty of Medicine, University of Helsinki, Helsinki, Finland; 16Department of Public Health and Welfare, Finnish Institute for Health and Welfare, Helsinki, Finland; 17École de Psychologie, Centre d’étude des troubles du sommeil, Centre de recherche CERVO/Brain Research Center, Université Laval, Québec, QC, Canada; 18Brain Institute, Physiology and Behavior Department, and Onofre Lopes University Hospital - Federal University of Rio Grande do Norte, Natal, Brazil; 19Helsinki Sleep Clinic, Terveystalo Healthcare, and Department of Neurosciences, Clinicum, University of Helsinki, Helsinki, Finland; 20IRCCS, Istituto delle Scienze Neurologiche di Bologna, Bologna, Italy; 21Department of Biomedical, Metabolic and Neural Sciences, University of Modena and Reggio Emilia, Modena, Italy; 22Sleep Medicine Center, Charité Universitätsmedizin Berlin, Berlin, Germany; 23Department of Emergency Medicine, Medical University of Gdansk, Gdansk, Poland; 24Department of Psychology, Sapienza University of Rome, Rome, Italy; 25Department of Psychiatry and Behavioral Sciences, Baylor College of Medicine, Houston, TX, USA; 26IRCCS Fondazione Santa Lucia, Rome, ItalyCorrespondence: Brigitte Holzinger, Tel +43 699 101 99 042, Email brigitte.holzinger@meduniwien.ac.atPurpose: The COVID-19 pandemic affects mental health and sleep, resulting in frequent nightmares. Therefore, identifying factors associated with nightmare frequency is important, as it can indicate mental health issues. The study aimed to investigate increases in nightmare frequency comparing the pre-pandemic and pandemic period, and identify its risk factors. Further, the mediating role of post-traumatic stress disorder symptoms between the pandemic and nightmares is explored.Patients and Methods: For this cross-sectional survey data were obtained via self-rating online survey (ICOSS: details in Partinen et al, 2021), which was open to anyone older than 18 years. The final volunteer sample consisted of 15,292 participants, divided according to their nightmare frequency (high: ≥ 1– 2 nights/week; low: < 1– 2 nights/week). A total of 9100 participants were excluded if answers on variables of interest were missing or receiving rewards for participation. Chi-square tests identified changes of nightmare frequency. Predictors of high nightmare frequency were assessed using logistic regression and presented as Odds Ratios. Post-hoc mediation models were used to investigate the role of post-traumatic stress symptoms (PTSS).Results: The mean age was 41.63 (SD=16.55) with 64.05% females. High nightmare frequency increased significantly from 13.24% to 22.35% during the pandemic. Factors associated with it included self-reported PTSS (OR=2.11), other mental disorders and various sleep disorders or problems. Financial burden due to the pandemic, confinement, having had COVID-19, and work situation during the pandemic were associated with nightmare frequency, those relations were partly mediated through PTSS.Conclusion: Our results display the pandemic influence on nightmare frequency, which in turn connects to multiple mental health and sleep factors. These relations were partly mediated through PTSS. The COVID-19 pandemic appears to have caused traumatization of a substantial proportion of society. Health care workers should consider nightmares in their screening routines, as it might indicate PTSS and/or other mental and sleep disorders.Keywords: COVID-19, nightmares, mental health, sleep, post-traumatic stress disorder, collective trauma