The striatum and the prefrontal cortex play an important role in cognitive time processing, and time perception depends on sustained attention. Narcolepsy patients are unable to maintain sustained attention, due probably to deficient hypocretin signalling. Impaired time perception has been found in Parkinson's disease (PD) and attributed to a dysfunctional dopaminergic striatal pacemaker. We aimed to assess time perception in patients with narcolepsy and PD and to compare the outcome to healthy control participants. Seventeen narcolepsy patients, 12 PD patients and 15 healthy controls performed a short time production task, where they had to produce an interval of 1, 2 or 5 s. The accuracy of time production differed significantly according to task target duration, and there was a trend towards a group difference with narcolepsy patients tending to overproduce all target durations. Absolute variability was significantly different between groups, with narcolepsy patients showing higher absolute variability in comparison to controls and PD patients. The analysis of the temporal course of time estimation showed more pronounced overproduction of each target duration at the end of each trial in narcolepsy patients, whereas performance was more or less stable in controls and PD patients. Overproduction and higher variability of all time durations in narcolepsy indicate impaired short interval timing in the seconds range, while the scalar property of timing was preserved. The time-course of accuracy and variability of time production within sessions indicate an attention-related mechanism of impaired interval timing.