Background: Obstructive sleep apnea syndrome (OSAS) and insomnia are two of the most common sleep disorders that coexist frequently having distinctive clinical features and therapeutic domain. Methodology: Patients presenting to our pulmonary medicine department with either the symptoms of OSAS or referred with risk factors were evaluated with insomnia severity index (ISI) and overnight polysomnography after a detailed history, clinical examination, calculation of pretest probability score and relevant pre-requisite workup. Results: Enrolled 100 patients were diagnosed case of OSAS, of these 65% were males and 35% were females. The mean age of the study group was 49.88 years (SD = 12.15). The youngest patient studied was 7 years old and the oldest was 74 years. The average BMI of our patients was 33.35 kg/m². Epworth sleepiness score (ESS) was normal in 24% of patients, 15% had mild, 25% had moderate, and 36% had severe excessive daytime sleepiness. As per the apnea-hypopnea index (AHI), mild OSAS was seen in 31%, 25% had moderate, and 44% had severe OSAS. The ISI revealed that 34% of patients had no clinically significant insomnia while 35% had subthreshold insomnia, 11% had moderate clinical insomnia, and 20% had severe clinical insomnia. A statistically significant association was seen between AHI and ESS, ISI with AHI, and ESS with ISI. Conclusion: ISI is a simple, reliable, and valid instrument to detect cases of insomnia and there is an increased prevalence of insomnia in OSAS patients. High prevalence leads to increased severity of OSAS and significant neurobehavioral morbidity and early detection aids in distinctive therapeutic domain.