Night-time sleep related cognitions have been shown to play a perpetuating role in insomnia. According to the cognitive model of insomnia day time cognitions (i.e. worry, rumination, etc.) may also contribute to it. The aim of this study was to investigate the possible role of daytime sleep-related rumination in Insomnia Disorder (n= 55, mean age 49.7±16.7 years), Obstructive Sleep Apnea Syndrome (OSAS) (n=33, mean age 58.1±10.2 years) and healthy subjects (n=33, mean age 49.8±13.9), using a set of sleep related variables which included the Daytime Insomnia Symptom Response Scale (DISRS), the Dysfunctional Beliefs about Sleep Scale (DBAS), the Pittsburgh Sleep Quality Index (PSQI) and the Insomnia Severity Index (ISI). Daytime sleep related rumination was higher in insomnia when compared to both OSAS (p<.001) and good sleepers (p<.001). In insomnia, elevated sleep related daytime rumination was best determined by unhelpful beliefs (coeff=0.3,p=.004), while in OSAS by insomnia symptoms (coeff=0.9,p=.02). These findings suggest that the association between insomnia-specific daytime rumination and unhelpful beliefs may be considered a cognitive feature of insomnia. In insomnia, sleep related cognition may dominate the 24-hour period. This finding might be of use for further investigations studying therapeutic strategies acting on cognitive processes to prevent and treat insomnia disorder.