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How Do Whales and Dolphins Sleep?

a dolphin swimming in a body of water

Sleep is one bodily function that just about every mammal has in common. It facilitates restoration and rejuvenation for the brain and body. Adult humans require 7 to 9 hours of sleep while adult lions sleep for 16 to 20 hours a day. However, for some mammals, sleep is not as straightforward as closing their eyes and clocking out for several hours.

Cetaceans are marine mammals and include dolphins, whales and porpoises. Like most mammals, they are viviparous (give live birth), are milk carriers to nurse their young, and are warm-blooded. However, what you may not expect is just like most mammals, they also require oxygen from the air rather than water. For this reason, these majestic creatures have adopted some interesting sleeping habits. Can you imagine having to sleepwalk every night to survive? Keep reading to find out how whales and dolphins have learned to adapt to breathing air while living in the ocean.

Why Don’t Whales and Dolphins Sleep Like Us?

Despite these cetaceans being aquatic, they are still mammals that require oxygen from the surface. For most mammals, breathing is a natural function that requires no conscious effort. As humans, we can reach full unconsciousness as we sleep because we will continue to breathe despite being unaware that we’re doing so. However, unlike most mammals, whales and dolphins are voluntary breathers. They have to think about every breath they take and consciously make the choice to breathe. If they were to fall fully unconscious, they would suffocate or drown.

In order to avoid certain death, whales and dolphins evolved to have several sleeping mechanisms that allow them to surface every now and then to breathe while getting enough sleep to survive. They do this by putting one-half of their brain to sleep. This is known as unihemispheric slow-wave sleep and is facilitated by the cetacean keeping the corresponding eye open. Having half of the brain active at all times allows cetaceans to have full control of their breathing and avoid predators and other potential dangers.

What Are the Mechanics of Sleeping Whales and Dolphins?

There are a variety of species of whales, dolphins, and porpoises, and they each have different methods of applying unihemispheric slow-wave sleep. There are a few similarities among species that include:

  • Sleeping with one eye open at a time.
  • Sleeping near the ocean’s surface.
  • Sleeping for short periods of time.

Mechanics of Sleeping Dolphins

Dolphins spend on average 33% of their day sleeping. They typically sleep at night for 2-4 hours at a time. Some species of dolphins go into a deeper kind of sleep where they rest either vertically or horizontally at the surface of the water. This is known as sleep logging because their still bodies resemble logs in the water. Other species of dolphins enter a lighter form of sleep where they continue moving slowly through the water. Some dolphins will even sleep near the sea floor if in shallow waters.

Dolphins typically sleep in pairs or large groups. This serves as a protective measure while they’re in one of their most vulnerable states.

When dolphins are first born, both the calf and its mother are required to swim constantly during their sleeping period for the first few weeks of the calf’s life. The young dolphin swims and sleeps in the mother’s slipstream, and this is known as Echelon swimming. This happens because young dolphins don’t have the proper body fat to float to the surface on their own for breathing or maintaining their body temperature. If the mother stops swimming during these first critical weeks, the calf could sink to the ocean floor and drown without its mother’s assistance.

Mechanics of Sleeping Whales

In contrast to dolphins, some whales spend much less time in their sleep state. Sperm whales, for example, only spend 7% of their day sleeping, and they only rest for 10 to 15 minutes at a time. Other species of whales, such as the grey whale, spend about 41% of the day sleeping. Whales also either sleep vertically or horizontally at the surface or slowly move through the water. They sleep in large groups at the surface and can sleep rather deeply. There are a few stories of boaters encountering pods of these sleeping creatures, and it appeared that they were unaware of a boat near them for several moments.

Young whales experience the same needs as young dolphins. They need to stay close to their mothers to help them avoid sinking to the depths and to maintain their body temperature.

a large body of water with a mountain in the background

What Are Other Ways Whales and Dolphins Facilitate Sleep?

In addition to unihemispheric slow-wave sleep, whales and dolphins have other biological features that contribute to their ability to maintain necessary sleep and breath behaviors. Their body fat allows them to easily surface, requiring much less energy and concentration. Some species of whales and dolphins have incredible lung capacities that let them hold their breath for up to an hour.

It may seem strange to us that animals that live in the largest bodies of water need to obtain oxygen in similar ways to land animals. After all, it has complicated one of the most basic and necessary functions of most living creatures. However, these cetaceans have creatively and strategically evolved in ways that allow them to thrive despite the inhospitable conditions. Unihemispheric slow-wave sleep may appear to be a hardship, but it allows these beautiful marine mammals to breathe while also giving them an edge in survival. By having parts of their brains awake at all times they have a higher chance of surviving predators and avoiding traps and dangers of the big blue.

If you’re interested in learning more about the wonder of cetaceans and seeing these majestic creatures in action firsthand, check out Next Level Sailing. We are the premier option for whale watching. If you’re interested in more than just whales and dolphins, you can also see other creatures of the deep up close. Contact us today to start your whale-watching journey.