Sleepiness is often neurophysiologically assessed using the multiple sleep latency test (MSLT) or the maintenance of wakefulness test (MWT). We examined the frequency of incidental intersession napping during MSLT and MWT testing to see if there was a relationship between intersession napping, mean sleep latency and subjective sleepiness on the Epworth Sleepiness Scale (ESS). We conducted a retrospective analysis of 24 studies of subjects who underwent either a MSLT or a MWT as a component of their clinical assessment and had coincidental wireless telemetry recording of their sleep in between scheduled naps. We found that 17.6% of the MSLT patients and 28.6% of the MWT patients slept inadvertently between test sessions. The group of patients who napped between sessions had shorter sleep latencies on the MSLT. No statistically significant group-wise difference between the sleep latencies of those who napped between MWT sessions and those who did not was found. There was no significant difference between the ESS of those who did and those who did not sleep between sessions. We found that brief inadvertent intersession napping was common during the MSLT and MWT, but there was no evidence to suggest that this significantly alters clinical test results.