The first finding of our study was that despite the fact that REM and non-REM sleep are functionally distinct sleep states, no differences in clinical presentation and sleepiness were found in our patients. Subjective and objective daytime sleepiness were consistently similar between REM and non-REM SDB patients, and complaints of sleepiness did not differentiate REM SDB. These results are partially in contrast with the first description of the disorder,1 in which the AHI during REM predicted the objective and subjective sleepiness of the patients, at least in patients with mild SDB. This could be explained by some differences in criteria applied to define REM SDB and in the assessment of sleepiness. Firstly, we considered patients with a wide range of SDB severity, while Kass et al1 examined only patients with clinically suspected SDB having AHI-TST < 10/h. In order to see if a population sample could explain the above differences, we extracted from our group patients with AHI-TST <10/h, and we applied the criteria of Kass and coworkers1 to compare REM-SDB, ie, patients with AHI-REM ≥ 15/h to non-REM SDB patients, ie, those with AHI-REM < 15/h. Sixty-six patients in our sample had AHI-TST <10/h, 53 cases defined as REM SDB and 13 cases defined as non-REM SDB according to criteria of Kass and coworkers.,1 Comparison between these subgroups confirmed the greater prevalence of women in the REM SDB group (50.9%), without, however, any differences for anthropometric, clinical, and polygraphic findings. Moreover, patients with AHI-REM ≥ 15/h were not more sleepy than patients with AHI-REM < 15/h, with no significant difference found for sleep latency at the MWT (p = 0.16), ESS score (p = 0.4), or the complaints of sleepiness (p = 0.6) or fatigue (p = 0.55). Thus we can conclude that the range of severity and the sample population alone could not explain the differences in the results. Second, while Kass et al1used the multiple sleep latency test, objective sleepiness was assessed by means of the MWT in our sample. Although the use of a different method to assess sleepiness could affect results, we believe that this is unlikely to be a factor contributing to different results. In fact, in line with our results, two extensive studies2–3 considering the influence of sleep stage on daytime sleepiness using the multiple sleep latency test have shown that sleep fragmentation limited to REM sleep has no specific impact on diurnal sleepiness. This is also confirmed by experimental data using acoustically induced clustered sleep fragmentation, which reproduces in some ways the sleep alterations found in REM SDB.13 In line with our results, the authors13 did not find any difference in daytime sleepiness, mood, and cognitive functions, suggesting that REM SDB should not be considered a specific sleep-state breathing disorder inducing greater sleepiness.