Searching for the daytime impairments of primary insomnia

Primary insomnia is a sleep disorder where the subjective complaint of initiating or maintaining sleep, or the experience of sleep that is non-refreshing, cannot be directly attributed to a comorbid medical or psychiatric disorder. For a diagnosis of primary insomnia, a patient must also report that the nighttime sleep disturbance is impacting upon daytime functioning. Yet, while subjective complaints of impaired wake-time functioning are well documented, consistent objective evidence of these impairments has proved elusive, particularly with regard to cognitive functioning. We aimed to review the body of literature examining neurobehavioural impairments in primary insomnia to identify which cognitive domains appear to be most consistently impaired in this group. The relatively few studies that have investigated neurobehavioural performance deficits in patients with primary insomnia have produced inconsistent and sometimes conflicting findings. It is suggested that methodological limitations, including heterogeneous test populations, variable testing protocols and conditions as well as unsuitable cognitive tasks have contributed to our inability to describe unequivocally the daytime impairments associated with insomnia. Based on our review, it appears that the deficits associated with insomnia are relatively subtle and may be qualitatively different to those that result from other sleep disorders and from imposed sleep deprivation. Attention tasks, which have a high cognitive load, and working memory tasks appear to show performance deficits more often than not in insomnia patients. It is important to more definitively characterise the daytime impairments associated with primary insomnia so that the efficacy of treatments to remedy the wake-time consequences of the disorder, in addition to the nighttime symptoms, can be investigated.