Physiologically and genetically, Syreen are Human: the only visible differences between the races are that Syreen skin tint is somewhat more bluish and their retinas reflect light like an Earth cat's. Additionally, the typical Syreen is more comfortable in a slightly warmer and more humid environment than the typical Human. According to some, Humans and Syreen have nearly identical diurnal cycles, exhibiting even further similarity between the two species. Other sources claim that the Human diurnal cycle differs greatly from that of the Syreen, which consists of twenty hours of wakefulness followed by ten hours of rest. By scanning Syra, the former variant can be confirmed.