## 4 Discussion
These findings indicate the importance of a waitlistcontrol group and could indicate regression to the mean, first night effects, or perhaps a learning effect where studies that draw participant attention to sleep variables may influence subjective responses over time, irrespective of treatment interventions. These results contrast with previous studies in insomnia samples that found evidence for a reduction of SOSD after CBT-I [14; 23; 25; 47; 48]. The lack of a CBT-I effect may be due to the shorter four-session CBT-I intervention used in the current study, as each of the above-mentioned studies included CBT-I programs with more than four sessions. Indeed, [PERSON] et al. (2021) [49] found no effect of a brief behavioural therapy for insomnia, versus active control on changes in total sleep time or wake after sleep onset SOSD from baseline to post-treatment (CBT-I)/waitlist control period [49]. Furthermore, [PERSON] et al. [46] investigated the effect of eight sessions of CBT-I for older adults with insomnia (N = 63) and compared this to a sample of older adults with good sleep who did not receive CBT-I (N = 51). [PERSON] et al. [46] found no group by time interaction on discrepancy, suggesting that CBT-I was not effective at reducing the SOSD in an older adult insomnia sample. It is possible that SOSD results partly from insomnia and partly for OSA-specific factors in people with COMISA [9]. This may explain the absence of an effect of CBT-I, versus control on reducing SOSD parameters. As this is the first study conducted that investigated the effectiveness of CBT-I versus control as a tool for reducing SOSD in COMISA, there have been no other studies looking at the timing of CBT-I and treatment of OSA. For example, future studies could compare groups that receive CBT-I before CPAP, CPAP before CBT-I, or CBT-I and CPAP simultaneously to determine which combination most effectively reduces SOSD. This would help address the question of the disorder-specific maintenance of SOSD in people with COMISA. Similarly to previous research with people with insomnia, we found few associations between baseline variables and SOSD in COMISA patients [18]. A small number of weak and inconsistent associations were observed between SOSD parameters and other questionnaire and sleep study parameters, including associations between higher stage-specific AHI (in N1, N2, and REM) and greater underestimation of sleep parameters. Given the exploratory nature of this cross-sectional aim and the large number of statistical tests conducted, it is likely that the handful of statistically significant correlations occurred due to type one error. This study supports evidence from studies of participants with insomnia alone [18] that baseline sociodemographic characteristics, sleep, and daytime impairments are not consistently associated with SOSD in individuals with COMISA. This study should be interpreted in light of some limitations. Firstly, this study only used four sessions of CBT-I, compared to the traditionally recommended 6-8 sessions of CBT-I. Previous studies have found CBT-I of more than four sessions to be effective at reducing SOSD in insomnia alone samples [14; 23; 25; 47; 48]. Thus, future studies should investigate the effect of 6-8 session CBT-I programs on SOSD in people with COMISA. Secondly, the focus of this study was on COMISA where potentially useful comparisons with insomnia alone or OSA alone groups were not available (e.g., [28]). Given the reported effectiveness of CBT-I in improving sleep estimates in patients with insomnia alone, future studies should compare the effect of CBT-I on SOSD between individuals with COMISA and those with insomnia alone, to investigate a potential moderation effect of co-morbid OSA on change in SOSD. CBT-I is clearly useful and beneficial to individuals with COMISA [31], but the results of this study suggest that a focus on SOSD reduction as an outcome when using CBT-I in individuals with COMISA may not be fruitful.