## 4 Discussion  
This is the first RCT to examine the effect of CBT-I, versus control, on subjective-objective sleep discrepancy in individuals with COMISA. At baseline, individuals with COMISA underestimated total sleep time by an average of 52 min, and overestimated sleep onset latency by 35 min compared to polysomnography measurements, with no consistent relationships between SOSD parameters and any sociodemographic, sleep, or daytime function measures. In this COMISA group it has previously been shown that CBT-I significantly improves (\(p\) < 0.001) insomnia severity (measured by the \"insomnia severity index\") (baseline M = 18.5, SD = 1.3; 6-weeks after CBT-I M = 12.2, SD = 1.4) compared to the waitlist control group (baseline M = 18.0, SD = 1.3; 6-weeks after control M = 16.6, SD = 1.4) [31]. However, unlike previous findings in those with insomnia alone, this improvement in sleep was not associated with a specific improvement in SOSD in people with COMISA. Previous studies investigating baseline SOSD in COMISA groups have reported inconsistent findings. Similar to [PERSON] et al. (2016) [29], we showed participants with COMISA significantly underestimated total sleep time and overestimated sleep onset latency before treatment [29]. However, our findings differ from other studies which have reported no or minimal subjective-objective discrepancy of sleep duration in COMISA [28,30]. This may be due to differences in study samples or methodology. For example, we recruited COMISA patients through specialist sleep clinics, while [PERSON] et al. (2021) [30] examined a population-based sample (N = 638). Furthermore, [PERSON] et al. (2013) [28] examined subjective-objective discrepancy during laboratory-based polysomnography. Contrastingly, our polysomnography studies were conducted in participants' home environment. This is more directly relevant to insomnia-related sleep perceptions, timing, and duration of objective recording, and discrepancy between parameters when compared to in laboratory studies [45]. [PERSON] et al. (2016) [29] reported overestimation of sleep onset latency in participants with insomnia alone (17.4 min), COMISA (14.5 min), and OSA alone (15 min). In contrast, a larger average sleep onset latency overestimation of 34.5 min was seen in our COMISA sample. [PERSON] et al. (2020) [34] found that among 355 patients with OSA, 35.5% overestimated sleep duration by 60 min or more, and 20% underestimated sleep duration by 60 min or more. In our COMISA sample, 9.9% of participants overestimated sleep duration by \(\geq\)60 min and 51.8% underestimated sleep duration by \(\geq\) 60 min suggestive of a potential tendency towards underestimating total sleep time in individuals with COMISA compared to individuals with OSA alone in other studies. Previous studies have reported that CBT-I reduces the discrepancy between subjective and objective measures of sleep in individuals with insomnia alone [14,23,25,46]. This is likely driven by a large improvement in subjective sleep estimates, and minimal change in objective sleep during treatment that results in a convergence of the two measures post CBT-I-treatment but could also be explained by regression to the mean [23,24]. [PERSON] et al. [14] and [PERSON] et al. [25] did not have control groups, and [PERSON] et al. [46] had a control group whereby the participants did not have insomnia (\"good sleepers\"), meaning that these studies were not RCTs with a sensible comparator group. Whereas the current study was a RCT and therefore it importantly controlled for temporal effects independent of CBT-I. The results of the current study show that total sleep time, sleep onset latency, and sleep efficiency, but not wake after sleep onset, SOSD declined over time with no differences between CBT-I and control groups.