"Sleep" is a common behavior found across the animal kingdom. It's a reversible and rhythmic state rooted in the ancient history of animal evolution, serving as a vital condition for survival.

Terrestrial animals have specific places for sleep, while aquatic fish, with their unique respiratory system, adapt to sleeping underwater. This behavior extends to cetaceans such as whales and dolphins.

For instance, like whales, cetaceans breathe with lungs and need to surface to breathe, posing a question: if they slept underwater, would they choke or suffocate?

Around 50 million years ago, the ancestors of aquatic mammals faced the challenge of sleeping underwater when they migrated to the sea. Overcoming obstacles, cetaceans, including whales, achieved a significant feat in evolutionary history.

Now, with a better understanding of cetacean sleep behavior, the question arises: How do these aquatic mammals sleep and breathe underwater?

At one point, a hypothesis suggested that large lungs in whales store oxygen, ensuring its availability during sleep. However, examining a fin whale's lung capacity, up to 2,000 liters, reveals it's less than half the percentage of lung capacity in a land mammal compared to its massive body.

Additionally, the lung capacity of cetacean mammals like dolphins is similar to terrestrial mammals, refuting the idea that large lungs simplify oxygen storage.

For protection, aquatic mammals expel most air from their lungs when diving, minimizing air exchange to prevent diving sickness.

So, how do whales ensure oxygen while sleeping underwater? The key lies in their respiratory principles:

1. Large Tidal Volumes: Whales have a tidal volume of 90% of their lung capacity, significantly more than the 15% seen in humans and terrestrial mammals. This ensures sufficient oxygen absorption during ventilation.

2. High Hemoglobin Concentration: Whales have high hemoglobin concentrations, the substance in blood carrying oxygen. This allows them to rapidly transport and utilize 90% of inhaled oxygen, whereas humans only use 25%.

3. Ultra-stable Myoglobin: Myoglobin, storing oxygen, is present in whales in an ultra-stable form, avoiding the drop in oxygen storage capacity seen in humans. Whales have 20 times more myoglobin than humans.

4. Fragmented Sleep Schedule: Unlike humans, animals don't need to sleep continuously. Sperm whales, for instance, engage in 10 to 20 minutes of sleep, surfacing to breathe in between. This behavior resembles taking a nap.

5. High CO2 Tolerance: Whales exhibit high tolerance to elevated carbon dioxide levels during dives, up to two and a half times the usual levels. This ensures they don't need to flare their nostrils to avoid choking on water during sleep.

In conclusion, adapting cetaceans to sleep underwater involves a combination of physiological features, allowing them to navigate the challenges of sleeping and breathing in their aquatic environment.