Diurnal animals produce sounds at night. In roosting species, high vocal activity at roosting sites may be a prerequisite to sleep, suggesting a role of chorusing in coordinating resting. In other species, vocalizations during sleep are commonly reported, which in humans correspond to dream contents. Dolphins’ nocturnal vocal activity has been rarely investigated. This animal model is interesting because: dolphin resting behaviour is associated with social synchrony (formation swimming/synchronous breathing), dolphins’ daily social activities are primarily mediated by vocalizations, and dolphins are known to mimic sounds of their environment. Therefore, we recorded the nocturnal vocal and breathing activities of a captive group of dolphins. The temporal pattern revealed two peaks of intense whistling, followed by a decrease in vocal activity and low respiration rates, resembling the presleep chorusing in other species. Within the night, we also found non-specific vocalizations, which appeared to be vocal copies of whale sounds broadcast during daily public shows. This suggests a vocal rehearsal of day salient events. These findings are questioning the significance of nocturnal vocal activity in dolphins as, contrarily to most previous reports, these productions are clearly outside a feeding context. They shed new light on the potential cognitive/social significance of auditory communication.