The possibility of using heart rate variability (HRV) as an indicator of sleepiness was investigated by analysing heart rate (HR) activity and electroencephalography (EEG) recordings from 10 individuals who performed a monotonous attention task for 120 min in both sleep-deprived and rested states. In both conditions, measurements were collected during 60 min of sleep immediately following a 120 min of non-sleep (awake phase). Although HR decreased significantly in both the rested and the sleep-deprived states during the awake phase, HR significantly changed sooner when subjects were sleep-deprived than when they were rested. No significant changes in HRV were found during the awake phase; however, HRV correlated significantly with alpha and theta power densities when rested but not when sleep-deprived. During the sleep phase, the total HRV and very low and low frequency HRV components significantly decreased approximately 40 min after sleeping in the sleep-deprived condition. These HRV components were also significantly and negatively correlated with delta power densities. HRV does not seem to be a viable indicator of sleepiness; however, HRV may be useful for determining sleep stages.