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Comorbid insomnia and sleep apnea (COMISA) is associated with poorer clinical outcomes than chronic insomnia disorder (CID) or obstructive sleep apnea (OSA) alone. However, the psychobehavioral characteristics that differentiate COMISA remain unclear. We aimed to examine the combined roles of dysfunctional beliefs and attitudes about sleep (DBAS) and excessive daytime sleepiness (EDS) in COMISA. We analyzed data from 283 patients (42 with CID alone, 166 with OSA alone, 75 with COMISA) from the National Center of Neurology and Psychiatry database, a component of a polysomnography databank for sleep disorders in Japan. DBAS was assessed using the 16-item DBAS scale (0-160 points), and EDS was measured using the Epworth Sleepiness Scale (ESS; 0-24 points). Participants were categorized into four groups based on median ESS and DBAS scale scores (medians: 90 and 9, respectively): low ESS/low DBAS, low ESS/high DBAS, high ESS/low DBAS, and high ESS/high DBAS. Logistic regression analyses, adjusting for multiple covariates, such as age, sex, and body mass index, were conducted. The high ESS/high DBAS phenotype was significantly associated with COMISA relative to the low ESS/low DBAS reference group in comparisons with both CID and OSA. The low ESS/high DBAS phenotype was also significantly associated with COMISA compared with OSA. Coexisting elevated DBAS and EDS appear to represent a distinct psychobehavioral phenotype distinguishing COMISA from CID or OSA alone. Assessing DBAS in patients with OSA and evaluating daytime sleepiness in those with CID may support earlier identification of COMISA and inform targeted management.