In Study 1, individuals with primary insomnia (n=32) and good sleepers (n=38), recruited from a university population, completed a semi-structured interview. The interview assessed monitoring for sleep-related threat, negative thoughts and safety behaviours, at night and during the day. Compared to good sleepers, individuals with insomnia reported more frequent monitoring at night and during the day, more negative thoughts associated with monitoring at night and during the day, and the use of more safety behaviours associated with monitoring at night. A path analysis showed that the total monitoring frequency was significantly positively related to the score for total negative thoughts, which in turn, was significantly positively related to the total number of safety behaviours used. Study 2 aimed to test the generalisability of these findings in a clinical sample of individuals with primary insomnia (n=23). The results indicated that the clinical sample also engaged in monitoring for sleep-related threat, experienced negative thoughts and used safety behaviours. Further, more severe insomnia was associated with more negative thoughts and the use of more safety behaviours associated with monitoring at night. Together, these results are consistent with the proposal that monitoring for sleep-related threat functions to maintain insomnia by triggering negative thoughts and the use of safety behaviours. Future research is required to verify the proposed causal status of monitoring, using prospective designs and experimental manipulations.