Working overtime is common in a few Asian countries in recent years, especially for the young working force. However, the productivity during overtime work has yet been scrutinized. In order to explore the changes in mental productivity when people are working overtime in the night, cognitive performances were investigated in this study. 8 healthy right-handed males were recruited and experienced three sleep-restriction conditions for 155 min, 225 min and 295 min, respectively. During sleep restrictions, subjects were required to watch documentary videos and complete three cognitive tasks. Thermal Sensation Vote (TSV), Stanford Sleepiness Scale (SSS), and NASA-TLX scales were administered to acquire the thermal sensation, perceived fatigue, and workload. Meanwhile, the skin temperature, core temperature and ECG data were continuously recorded. Results showed that the sleepiness and workload during these three sleep-restriction conditions were significantly higher than the baseline level. Meanwhile, both the mean skin temperature and core temperature showed similar patterns, and monotonous heart rate reduction was observed, during all these three sleep-restriction conditions. In addition, time-domain index of Heart Rate Variability (HRV): the Root Mean Square of Successive Differences (RMSSD) increases were observed during the early stage of sleep restrictions, followed by several fluctuations in later stages. Furthermore, performance index (PI) of all three tasks and the overall performance index (OPI) showed similar patterns. Specifically, they reduced during all three sleep-restriction conditions, during which several fluctuations were observed, suggesting that cognitive performances were compromised by sleep restrictions. However, fluctuations of PI also showed different patterns, which was depended on task difficulties, suggesting that cognitive performances were affected differently by task loads. The reason for both PI fluctuations and OPI fluctuations could be ascribed to the compensatory effects and the adaptability of the human body to stimuli (sleep restrictions), accompanied by temporarily rising sympathetic activities and arousal levels. Findings of this study revealed the changes of cognitive performances during sleep restrictions, which could provide references for the optimization of working schedules and arrangements during overtime work in the night.