This study aimed to investigate the existence of day-night differences in the time for anesthesia and recovery in tambaqui exposed to the anesthetic eugenol and the influence of feeding time. Thus, we evaluated: (1) swimming activity; (2) food anticipatory activity (FAA) as a synchronizer of swimming activity and change to susceptibility to anesthetic; and (3) the effects of diurnal/nocturnal anesthesia exposure of fish feeding in the mid-light phase: 12:00 h (ML) and fish feeding in the mid-dark phase: 00:00 h (MD). Our findings revealed strictly nocturnal activity for tambaqui (94.2%), known as diurnal fish to date. Moreover, FAA was observed in tambaqui fed at MD, which showed a sustained increase in activity that began 2 h before feeding time and lasted until feeding. In contrast, no FAA was observed in fish fed at ML. Regarding anesthesia by day or night, the tambaqui treated with eugenol exhibited no difference in induction time. However, differences were observed in recovery times, with fish anesthetized at day recovering in 1-2 min and fish anesthetized at night recovering in 5-7 min. In short, our findings revealed for the first time the nocturnal behavior of tambaqui. These results indicated that recovery by day/night by eugenol in tambaqui has a strong dependence of behavioral patterns and the time of day.