Background: The reported prevalence of insomnia symptoms in people with intellectual disabilities varies greatly, possibly due to the lack of a common definition. This article provides an overview of the different definitions used and formulates key points for a general definition. Methods: A literature search was performed. An overview of the definitions used was given and compared to the third edition of the international classification of sleep disorders. Results: The search yielded 16 studies. No uniform definition was used. Terminology and cut-off points of insomnia symptoms differed. Insomnia symptoms were mostly described as night-time problems. A minority of studies incorporated daytime consequences. Conclusion: An insomnia disorder entails more than merely night-time complaints and should include daytime consequences. A general definition is warranted. This definition should focus on night-time and daytime insomnia symptoms, incorporate subjective features, and discuss the use of objective measurements and influence of environmental circumstances.