The current study aimed to explore the potential occurrence of the third-quarter phenomenon among analog space mission participants. While the phenomenon is quite often referenced in the literature, many findings are anecdotal or have methodological limitations. To address them, we conducted our study in a standardized environment of the LunAres Research Station in Piła, Poland. We collected data from 88 analog astronauts participating in sixteen two-week missions. We examined the third-quarter phenomenon, focusing on positive and negative emotions, overall emotional positivity, and task performance. We observed a distinct decrease in emotional positivity on the sixth day of the mission, with the lowest intensity of positive emotions and the highest intensity of negative emotions. Though fluctuating, task performance improved throughout the missions, with no third-quarter phenomenon pattern. We concluded that our study does not support the third-quarter phenomenon and suggests that negative emotions and outcomes might occur during different mission periods. Hence, the possibility of such variations in the analog astronauts’ emotions and activities should be considered when training future astronauts in the analog space missions.