Tuna swim when they sleep, because they need to swim all the time to breathe normally, so tuna will keep moving even when they sleep. In addition, tuna has no eyelids, so it is impossible to tell whether they are sleeping by their eyes. However, there is still a big difference between tuna and peacetime after entering a state of rest. Under normal circumstances, tuna swimming speed is relatively fast, when they sleep, swimming speed will drop. In addition, their metabolism will also decline and their energy consumption will also decrease.

Tuna swim all the time because their gills have degenerated, and they need constant activities to let the water flow through the gills, so that they can get enough oxygen. If they don't swim, they are likely to suffocate and die, so tuna will keep swimming in any state, even when sleeping, so as to avoid the lack of oxygen in tuna. However, they sleep for a short time, and they will wake up soon after sleeping, so they are not easy to be observed when they sleep.