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How do fish sleep? And turtles? And whales?

Updated: Sep 3, 2021

How do whales sleep?
Sleeping Sperm Whales (by Getty images)

Ever looked at a still fish underwater and wondered if it was asleep? So do we!

Some of the mysteries of how fish and other marine animals sleep are revealed here. Read on and don't snooze!

1. Fish

To determine whether and how fish sleep, scientists first had to determine what sleep is for fish. In humans, it is seen as a period of rest accompanied by closed eyes and a change in brain activity (the neocortex). However, most fish do not have eyelids or a neocortex. So scientists started observing behavioral sleep patterns - periods of resting compared to periods of activity - and found that fish do indeed sleep. Certain fish, like sharks or damsel fish, need to stay quite active during their resting period to ventilate their gills and keep breathing - damsel fish hide in the coral but keep sweeping water over their gills with their fins, sharks never stop swimming which has gained them the reputation of never sleeping (but they do actually rest).

A lot of the studies on brain activity during rest in fish have been done on zebra fish who have translucent skin on their head allowing scientists to observe their brain activity with a microscope during sleep without disturbing them!

Findings have shown that fish, like humans can suffer from insomnia and disrupted sleep patterns, naturally or for instance when they have eggs to look after(the early stages of parenthood), or when they are separated from their group and invert their sleep patterns to stay safe (staying awake at night where they have less risk of being found by predators for instance). Furthermore, if the lighting cycle changes or their pattern is disturbed, they may have a longer sleep during the next resting period to catch up.

Most fish lie on the bottom , tail on the ground, when in a resting position; some may hide in crevices or caves. Some, like certain species of parrot fish and wrasse, secrete a mucous membrane that acts as a sleeping bag/mosquito net at night. Scientists first believed that this was to screen the fish from predators but new studies show it is more likely a defense against parasites like ocean isopods or sea lice.

When on your next Night dive, have a peek under rocks to see them nestled in their slimy sleeping bags. But don't shine your light in their face, you'll upset their sleeping pattern and they'll need a lie-in the next day !

2. Sea Turtles

During one of your dives on the east coast of Nusa penida, you might catch a hawksbill or green turtles catching a few winks under a rock ledge or nestled under the outcrop of a coral bommie. If you are lucky enough to see hatchling turtles floating on the surface you might see them resting in their favourite position with their front flippers folded over their backs. Adults can also sleep like this.

What allows Sea turtles to sleep on the bottom is their ability to hold their breath for several hours at a time and the slowing of their metabolic rate and, thus, oxygen use, when resting. This way, they can stay submerged for several hours without needing to come up for air.

Different species have different sleeping styles. Hawksbills and Green often return to the same sleeping spot. They tend to like rock ledges and snuggling up under small coral outcrops. Olive Ridleys like basking in the sun, on the beach or floating on the surface; it is not unusual to see hundreds of Olive Ridleys floating on the surface in front of resting beaches. Flatsbacks have also been observed floating on the surface, sometimes with sea birds perched on their backs

3. Whales and dolphins

Whales have been observed floating on the surface sinking down a meter or so then popping up after a few minutes to breath, they have also been seen floating vertical under a few meters of water but again they come up every so often to breath. So how do they remember to breathe ? Unlike us, cetacean ( and other marine mammals) breathing is not automatic – not an involuntary autonomic response as scientists call it- like for humans. It is voluntary, meaning the whales actually have to make a conscious decision to breathe, not easily done when fast asleep.

Indeed, half the brain is in a state of awareness whilst the other rests in what is called deep sleep or slow wave sleep. There are two different types of sleep, REM ( Rapid Eye Movement) sleep and non REM sleep. In non-REM sleep, the deepest sleep is slow wave sleep, where the electrical impulses are least frequent, that is when memories are consolidated and essential repairs are done to the bodily systems.

Conclusive studies have been done on Bottle-nose dolphins showing each side of their brain gets approximately 4 hours sleep in short stints in a 24 hours period. Half the brain goes into deep sleep necessary to the animal's survival, whilst the other part of the brain manages breathing and predators, also essential to the animal's survival. Then they swap. It is called Uni hemispheric Slow Wave Sleep (USWS). Surprisingly , though studies have shown that seals have REM sleep, which is the period during which you dream, Cetaceans have been shown to not experience any REM sleep so many scientists hypothesize that they do not dream.

What is certain is that all creatures seem to rest , they have a time when their bodies rest and repair, albeit is all sorts of different ways. Certain species of sharks sleep whilst they swim, and whales and dolphins are only ever half asleep and yet they too rest in Morpheus's arms in the land of slumber, just like we do.

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