Why did the shark turn over and fall asleep?

This behavior of sharks is called "tension immobility" (TI), which is an unconditional reflex behavior. Nervous sharks will behave immobile and unresponsive. Many animals have this behavior, including insects, amphibians, reptiles, birds, mammals and fish.

This is an effective way to subdue sharks, that is, to put them into a rigid and static state. Samuel gruber of Bimini Shark Laboratory discovered this when he was studying the lemon shark in Bimini mangrove forest. When the shark is turned upside down, it will enter a rigid and static state, stop struggling and stay still.

Many kinds of sharks will have TI behavior when turning over, such as baiji, lemon shark, high fin shark, cat shark, whitetip shark, black fin shark and so on. It is said that this unnatural posture will cause changes in the animal's perception-movement system, resulting in a state similar to numbness and paralysis. In this state, the animal's muscle tension will drop, and it will begin to take a rhythmic deep breath. Sharks usually take less than a minute to perform well. Sharks can stay in this state for as long as 15 minutes.

There are also some sharks that can show TI behavior in other ways. For example, ferret sharks, as long as they gently put their hands on the lower part of the shark's nose (the position of Lauren's ampulla), will have TI behavior. Lauren ampulla is the sensory organ of shark, which may change the electromagnetic induction ability of shark and lead to TI behavior.