How Do Fish Sleep?
Sleep plays an essential role in our overall functioning, development, and how long we live. Without enough sleep, our health and cognitive performance suffers. The same goes for animals, including fish.
While nearly all animals sleep, the way they sleep can be very different, especially in the case of fish. Fish sleep looks so different that many researchers prefer to call it rest instead of sleep.
Do Fish Sleep?
Many types of fish appear to sleep, but fish sleep differs from what we usually think of as sleep. Researchers have not been able to measure the familiar brain wave patterns that characterize human sleep and the sleep of many other animals in most fish, so researchers often refer to fish sleep as rest. Also, since most fish do not have eyelids, they can’t close their eyes during this rest.
Fish appear to be more alert than humans are during sleep, which may give them more time to react to potential threats in their environment. Still, fish do slow down metabolic processes in their resting state, much like humans. They physically slow down as well, with some fish floating in place.
How Do Researchers Know That a Fish Is Sleeping?
Since fish do not look very different when they are asleep, it can be difficult to determine whether a fish is resting or awake. However, during a state of rest, fish are much less responsive. Some may appear to stop moving completely, and can even be touched or handled without waking up.
Moreover, some fish are more vulnerable to being attacked by predators at night, which suggests that they are diurnal, or resting and less alert at night. Studies have found that certain fish species experience more deaths in the first two hours after sunset, which indicates that these fish tend to fall asleep at that time.
A lack of responsiveness is often a sign of fish sleep, but researchers have found additional markers of sleep that fish share with humans and other animals. For example, studies show that melatonin may regulate the sleep-wake cycles in zebrafish. Zebrafish also appear to have distinct stages of sleep like humans, and follow a circadian rhythm.
The Stages of Fish Sleep
Humans cycle through four stages of sleep, including two stages of light sleep, one stage of slow-wave, deep sleep, and one stage of rapid eye movement (REM) sleep. Although fish do not have a neocortex like humans, researchers were able to monitor the brain activity, heart rate, and eye and muscle movement in zebrafish to document two stages of sleep that are similar to slow-wave sleep and REM sleep.
When fish do not get enough sleep, they seem to be vulnerable to the effects of sleep deprivation. After a night of sleep deprivation, zebrafish experience a sleep rebound, in which they catch up on sleep.
This resting state is so essential to fish that they will find ways to maintain a bare minimum of sleep, just like other animals. For example, if fish are subjected to experiments in which their sleep is interrupted, they will eventually nod off into microsleeps, managing to obtain at least 5% to 10% of their normal amount of sleep.
1. predator n. 捕食者
2. diurnal adj. 日间活动的
3. melatonin n. 褪黑素
4. circadian adj. （指每24小时人或动物体内变化）昼夜节律的，生理节奏的
5. neocortex n. （大脑）新皮质
6. deprivation n. 缺乏；剥夺
7. rebound n. 反弹