Do fish sleep at all

Like many vertebrates and other living beings, fish also sleep. As they do not have a societal pattern of living, they go to sleep whenever they are tired or whenever there is no need or scope for other normal wakeful activities. Since they also rely on light to see events and objects, they usually resort to sleep during nights.

More than from other signs, we understand whether a man is asleep or awake based on the fact whether the eyelids are blinking or session-closed. Such an inference is not possible with fish because they have transparent eyelids which are mostly kept closed, even when they are awake, as shields to protect the sensitive eyeballs.

A fish undertakes sleep either by suspending itself in quiet water or by resting on the floor bed or under logs or hard objects if it is current water. Suspended steady state in water or inactive stay on the floor bed, however, is not a sure indication that the fish are asleep. Then, how does one know whether the fish are asleep or awake?

There are a few additional visual observations based on which we can know whether the fish are asleep or awake. A fish performs its respiration through its gills with its mouth and jaws assisting in tandem.

It takes water copiously through the mouth and drains it by pressing its jaws through its gills. The gills feature a thick comb-like framework that holds a dense matrix of blood capillaries. There, the dissolved oxygen in the flowing water is exchanged en-route with the carbon dioxide through the walls of the blood capillaries.

To have sufficient water between its jaws so as to ensure good surge over the gills by pressing the jaws, the fish first closes the gill covers when it takes in water and opens the gill covers while closing the mouth before it and next, presses the jaws to allow the water to flow over the gills.

In other words, the opening and closing of mouth are out of phase with the opening and closing of gill covers. This asynchronous activity takes place at a moderate frequency when the fish is awake. During sleep, the frequency of this mouth-jaw-gill-mouth sequential (respiratory) cycle is rather low. The ratio of these two (wakeful and asleep respiratory) frequencies, however, varies from species to species of the fish.