This thesis aimed to compare the disturbances in sleep, daytime functioning, and quality of life associated with insomnia with a comorbid psychiatric or medical disorder to those of insomnia alone. First, a review of the literature on the sleep disturbances and daytime impairments characterizing comorbid insomnia as compared to insomnia alone was conducted. The review included 38 studies and revealed that insomnia with a comorbid psychiatric disorder was not associated with more severe subjective sleep disturbances than insomnia alone, but that it was related to increased night-to-night sleep variability. Insomnia comorbid with a depressive disorder involved a higher proportion of rapid eye movement (REM) sleep and less high frequency electroencephalographic activity during the night than insomnia alone. Insomnia with a comorbid psychiatric (mainly depressive) disorder was linked to higher health care use and more severe daytime impairments in mood, cognition, and ability to complete daily activities. Few differences in sleep were observed between insomnia with and insomnia without comorbid chronic pain. The rate and severity of mood disturbances and risks of sick leave and disability were higher when insomnia was accompanied by pain. Among reviewed studies, most investigated insomnia with a comorbid depressive disorder or painful condition. A limited number explored medical comorbidity and those that compared objective sleep disturbances in insomnia with and without a comorbid psychiatric disorder yielded inconclusive results. Many studies had major methodological limitations, including the use of different criteria to define insomnia for subgroups within a same study and the lack of control for the confounding effects of other comorbid disorders and prescribed medications. Considering these limitations, an empirical study comparing the sleep disturbances and daytime impairments characterizing insomnia with a comorbid psychiatric disorder and insomnia with a comorbid medical disorder to those of insomnia alone was conducted. The sample included 84 adults, divided into five groups: insomnia alone (INS; n = 22), insomnia comorbid with a psychiatric disorder (INS+PSY; n = 16), insomnia comorbid with a medical disorder (INS+MED; n = 14), good sleepers with a medical disorder (GS+MED; n = 17), and healthy good sleepers (n = 15). Multivariate analyses followed by canonical correlations for a priori contrasts revealed the INS+PSY group reported more nights of nonrestorative sleep and more severe daytime impairments (motivation, physical fatigue, physical and occupational functioning, insomnia-related consequences) than the INS group. According to sleep diaries, participants in the INS+MED group had shorter nights of sleep and more nighttime awakenings than those in the INS group. The former were also less productive at work. Compared to GS+MED, the INS+MED group was more depressed and fatigued, and had poorer occupational functioning, while the INS group had better physical functioning, but more severe depressive symptoms, higher levels of fatigue, and less vitality. Results of the thesis suggest that the nature of sleep complaints differs in insomnia with and without a comorbid depressive disorder and that insomnia with a comorbid psychiatric disorder is characterized by more severe daytime deficits compared to insomnia alone. Insomnia with a comorbid medical disorder was associated with a shorter and more fragmented sleep, and with more impairments in mood and role functioning than insomnia alone. Findings raise the possibilities that insomnia with a comorbid psychiatric disorder is distinct from insomnia alone and that psychiatric comorbidity contributes to the maintenance of insomnia via transdiagnostic cognitive and behavioral processes (e.g., repetitive thinking). Results support the relevance of adapting interventions for insomnia among persons with another disorder, notably a psychiatric disorder, to help them overcome the burden of a dual diagnosis and insure improved sleep and well-being.