Two experiments that aimed to investigate the association between clock monitoring, pre-sleep worry and sleep are presented. In Experiment 1, 30 good and 30 poor sleepers were instructed either to monitor or not to monitor a clock as they were trying to get to sleep. Worry was indexed by self-rating. Sleep was measured by self-report and actigraphy. Compared to non-monitors, clock-monitors reported more pre-sleep worry and they experienced longer sleep onset latency (SOL). These findings held true for both good and poor sleepers. In Experiment 2, following one night of baseline measurement, 38 individuals diagnosed with primary insomnia were instructed to monitor either a clock or a digit display unit (a control monitoring task) as they were trying to get to sleep. The clock-monitoring task was rated to be more worry provoking and sleep interfering than the display unit-monitoring task. Whilst display unit-monitors experienced less pre-sleep worry, the clock-monitors experienced more pre-sleep worry and reported a longer SOL on the experimental night, relative to baseline. Further, compared to the display unit-monitors, the clock-monitors overestimated their SOL more on the experimental night. Together, these findings suggest that clock monitoring may trigger pre-sleep worry and serve to maintain insomnia by fuelling pre-sleep worry and exacerbating misperception of sleep.