This study investigated how age, sex, and ethnicity relate to the severity of periodic limb movement index (PLMI) and obstructive sleep apnea (OSA) in a diverse clinical population. A retrospective analysis was conducted on 711 adults who underwent diagnostic polysomnography between June 2022 and July 2024. The mean age was 57.2 years; 55.6% were female, and the sample was ethnically diverse (53.7% White, 31.6% Hispanic, 6.8% Asian, 6.1% Black, 1.8% Other). PLMI was significantly higher in older adults, males, and White participants. AHI was higher in males and peaked in middle-aged adults (44–62 years). Multivariable regression models showed that male sex independently predicted higher PLMI and AHI, while increasing age was associated with higher PLMI only. Black ethnicity was linked to lower PLMI, and Hispanic ethnicity to higher AHI. These findings emphasize the role of demographic factors in the presentation of sleep disorders and support the need for personalized approaches in screening and diagnosis. Recognizing at-risk subgroups may improve early detection and targeted interventions for both PLMS and OSA in diverse populations.