There is a difference between the state of sleep and wakefulness. An animal in consciousness is characterized by brain activity and awareness. When the body enters sleep, many of its reactions slow down.
After a series of studies in the mid-2000s, it was shown that the transition from sleep to wakefulness occurs by neurons located in the hypothalamus. Hypothalamic neurons secrete a substance – orexin (in other words – hypocretin). It is a neuropeptide that regulates wakefulness. Too little orexin leads to conditions such as narcolepsy.
In subsequent studies, it was shown that fish also possess this neuronal nucleus, which suggests that they have physiological possibilities for sleep.
How do fish sleep?
While we have evidence that fish can sleep, it is still difficult to tell if they do. In mammals and birds, sleep studies use an electroencephalogram – a test of brain activity obtained by stimulating the cerebral cortex, which is absent in fish. And the use of an electroencephalogram in an aquatic environment is almost impossible.
However, there are other criteria by which we can find out. There are certain sleep patterns worth paying attention to, including:
- Prolonged inactivity. The fish remains motionless for a long time.
- Cover use. When the fish are resting, they seek refuge. It could be a small cave, a gorge under a rock, or even in the sand at the bottom of the ocean.
- Decreased sensitivity. While fish are sleeping, they become less sensitive to environmental stimuli.
When fish enter a sleep state, their metabolic rate, heart rate, and respiration rate decrease.
When do fish sleep?
There are over 33,000 fish species in the world. Some of them are nocturnal, others are active during the day or at dusk.
For example, Mozambican tilapias ( Oreochromis mossambicus ) sleep at night, descending to the seabed. Their eyes freeze during sleep and breathing slows down.
At the same time, the American catfish ( Ameiurus nebulosus ) is nocturnal, spending the day in the shelter. During sleep, he reacts poorly to sound and contact stimuli, his breathing and pulse slow down.
Tench ( Tinca tinca ) is another nocturnal fish species. Wanting to take a nap, he floats to the bottom and “turns off” for a full 20 minutes.
Usually fish do not sleep for a long time: they spend several minutes sleeping. This is a safety issue as long periods of sleep make fish vulnerable to predators.
How can you sleep with your eyes open?
There is a myth that fish cannot sleep because they do not close their eyes. They do not actually close them, only this has nothing to do with sleep: the fish simply do not have eyelids. However, there are exceptions. Some shark species have a third eyelid – the nictitating membrane, which is preserved in a rudimentary form in many mammals and even in some human people. This membrane protects the eyes of sharks, but they also do not cover it for sleep.
Also, sharks, unlike other fish, cannot stop for rest: when moving, water passes through the gills – this is how they breathe. This is why sharks continue to swim even while sleeping, albeit slower than usual.