SummaryInappropriate sleep‐related beliefs and behaviours are considered key maladaptive mechanisms in the development and maintenance of insomnia in the otherwise healthy population. The aim of this study was to evaluate critically the role of sleep‐related beliefs and behaviours in insomnia after acquired brain injury. Cross‐sectional data of 51 outpatients with insomnia disorder and acquired brain injury were used to evaluate associations of the insomnia severity index with the dysfunctional beliefs and attitudes about sleep scale and sleep‐related behaviours questionnaire. Seven (44%) of the dysfunctional beliefs and attitudes about sleep scale items and 10 (31%) of the sleep‐related behaviours questionnaire items correlated significantly with insomnia severity. Ten experts were consulted on whether they considered the questionnaire items maladaptive or accurately reflecting coping with conditions experienced by people with acquired brain injury. Although multiple linear regression showed that the total scores of the questionnaires explained a significant part of interindividual differences in insomnia severity (R² = 0.27, F(2,48) = 8.72, p < 0.01), the experts unanimously rated only four (25%) of the dysfunctional beliefs and attitudes about sleep scale items as dysfunctional beliefs and three (9%) of the sleep‐related behaviours questionnaire items as safety behaviours. In people with brain injury, sleep related beliefs and behaviours may also play a role in insomnia, especially a diminished perception of control and worry about sleep. However, more than half of the questionnaire items on sleep‐related beliefs and behaviours may not be considered inappropriate and maladaptive for the acquired brain injury population, and may reflect adequate observations and efforts in coping with consequences of the brain damage.