Long term patterns of heart rate variability (HRV) features were decreased in subjects with higher self reporting stress scores. For mobile applications, short term analysis of HRV features may be ideal since conventional heartbeat recordings (3~5 min) might be inadequately long. In this study, short term analysis has been performed for heartbeat data obtained at five different time points from two subject groups (15 under high and 18 under low mental stress). The reliability of short term heartbeat data was demonstrated by detecting significant differences in long term patterns of HRV features between two groups. Fifteen to thirty second heartbeat measurements were long enough to produce reliable long term patterns of HRV features. Thus, short and intermittent recordings of heartbeats could be used to detect long term HRV patterns and offer a convenient method to monitor mental stress in mobile environments.