KininarunoteA notebook of curious things

This site is only a doorway

Everything here is gathered from things other people actually researched, thought through, and built: books, papers, historical records, and the people behind them. If something here catches your interest, go look at the original sources listed at the bottom of the page. They carry more depth than a page like this ever can.

Why Don't Dolphins Drown When They Sleep? How Half-Brain Sleep Works

Category: Living Things

In the late afternoon at an aquarium, you might see a dolphin swimming slowly in the pool. If you look closely, you may notice that one eye is closed. It seems strange: if a dolphin fell asleep, wouldn't it stop breathing and drown? What is happening in a dolphin’s brain? This article explores the mechanism of sleeping with half the brain and examines how certain the old explanation—that they do it to breathe—really is, based on scientific research.

1. Dolphins Sleep With One Brain Hemisphere at a Time and One Eye Closed

When researchers study the sleep of dolphins and whales using brain waves, they find that only one hemisphere of the brain produces deep sleep waves, while the other hemisphere produces waves similar to when the animal is awake. This is called unihemispheric sleep [1]. The eyes are connected to the brain in a crossed pattern. When the right hemisphere sleeps, the left eye closes or remains half-open. In records of beluga whales and bottlenose dolphins, the eye opposite the awake hemisphere was open for 95–98% of the time [2]. Another research team notes that these deep sleep waves are always linked to the closing of the opposite eye, and captive dolphins are considered to be sleeping if at least one eye is closed [4]. It is like turning off the light in one room of a house while keeping the other room lit. However, the actual brain does not switch like a simple on/off button.

2. Most Whale and Dolphin Sleep Is Half-Brain Sleep While Swimming

Records indicate that 70–90% of sleep in cetaceans (whales and dolphins) is unihemispheric sleep [2]. This does not mean they sleep 70–90% of the day; it means 70–90% of their sleep time is unihemispheric sleep. The state where deep sleep waves appear in both hemispheres simultaneously has not been recorded under normal conditions. Unihemispheric sleep can occur while swimming slowly, floating at the surface, or resting on the bottom of a pool. However, for bottlenose dolphins resting on the pool bottom, records show they briefly wake up every time they surface to breathe [2]. For captive bottlenose dolphins, one period of unihemispheric sleep lasted from 4 to 132 minutes, with an average of 42 minutes. These are experimental values from captive animals, not fixed numbers for wild dolphins. It is difficult to measure brain waves in the wild, so most records come from captivity [2].

3. Do Dolphins Have to Think About Breathing?

The idea that “dolphins breathe voluntarily, so if they sleep, they will drown” came from a suggestion made in 1964 by researcher Lilly, who speculated that breathing is "almost, if not completely, voluntary" [2]. However, in 1969, researcher McCormick reported that dolphin breathing is also driven by the body's system that senses carbon dioxide and oxygen levels in the blood. This means the body makes them breathe even without conscious effort [2]. Later, experiments used anesthesia to put both brain hemispheres into deep sleep. The dolphins' breathing stopped, and they would die without artificial respiration. From this, Mukhametov and colleagues concluded that "at least one hemisphere must be active to maintain breathing" [2]. Yet, a current review states there is no evidence that the awake hemisphere controls the brainstem's breathing circuits during sleep. This is why we cannot definitively say that unihemispheric sleep evolved for the sake of breathing [2].

4. Scientists Still Do Not Know Why Dolphins Sleep Half-Brained

There are several explanations for the role of unihemispheric sleep. The awake hemisphere may handle swimming and surfacing to breathe. The open eye may watch for approaching predators. Moving fins and tails keeps the body near the surface and warm [1]. These are explanations that "might be true," not proven reasons. As seen in the previous section, there are cautious views on the idea that breathing is the main reason. Why unihemispheric sleep evolved has not yet been settled among researchers [1].

5. Do Newborn Dolphins and Killer Whales Go Without Sleep?

In 2005, Lyamin and colleagues reported in Nature magazine on newborn killer whales and bottlenose dolphins and their mothers. They observed that for about a month after birth, they showed little or no typical sleep behavior of staying still and resting. They kept moving for 24 hours, avoiding obstacles. Rest time increased to adult levels over several months but did not exceed them [3]. This was an observation of "no typical sleep behavior," not an assertion that they "do not sleep." The following year, in 2006, another research team published a rebuttal in the same journal. Observing three pairs of bottlenose dolphins, they found that both calves and mothers took "swim rest" (resting while swimming slowly) for about 25–41% of the observation time. Underwater, they closed one or both eyes, but opened their eyes when surfacing to breathe [4]. They pointed out that looking only at eye status at the surface can miss rest occurring underwater. The figures represent the range from two pairs studied for calves [4].

6. A Listening Experiment Tested Dolphins for Up to 15 Days

In a US study, two bottlenose dolphins performed a task where they used echolocation to find and report randomly sounding target tones. A 2006 experiment lasted 5 days (120 hours). A 2012 paper described a 5-day experiment with two dolphins, followed by an experiment with one dolphin lasting at least 15 days, during which they continued to report accurately without interruption [5]. In the 5-day experiment, there were no signs of sleep deprivation in behavior or blood indicators, nor any obvious rebound sleep afterward. However, the lack of rebound sleep was only observed in the 5-day experiment. The 15-day paper did not measure sleep itself. The authors also noted that evidence was insufficient to show that unihemispheric sleep supported this long period of vigilance [5].

7. How to Watch Dolphin Eyes and Swimming at an Aquarium

If you have a chance to see dolphins at an aquarium, pay attention to their eyes and swimming style. Is there an individual with one eye closed? Is it the right or left eye? What kind of swimming are they doing at that moment? What happens to their eyes when they surface to breathe? You cannot determine if they are sleeping based on eye status alone. However, this can be a clue to verify what you have read in this article with your own eyes. Do not tap the glass or use flash photography. Please observe while following the facility's rules. If you want to learn more, reading explanations from aquarium staff or official website descriptions is a good method.

Sources

  1. Scientific American, "Meet the Animals That Literally Sleep with One Eye Open" https://www.scientificamerican.com/article/meet-the-animals-that-literally-sleep-with-one-eye-open/ (Overview of unihemispheric sleep and proposed functions.)
  2. Lyamin & Siegel, "Sleep in Aquatic Mammals" (Handbook of Behavioral Neuroscience) https://www.semel.ucla.edu/wp-content/uploads/2026/01/19_aquatic_mammals_proof_lyamin_siegel.pdf (Details on sleep ratios, eye closure, duration, and the history of breathing theories.)
  3. Lyamin et al., Nature 435, 1177 (2005) https://teams.semel.ucla.edu/sites/default/files/sleep/pdf/05-postpartum-cetacean-nature.pdf (Observations of newborn and mother cetaceans.)
  4. "Sleep in continuously active dolphins," Nature 441, E9 (2006) https://www.nature.com/articles/nature04898 (Rebuttal to the 2005 report regarding dolphin sleep.)
  5. Branstetter et al., PLoS ONE (2012) https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0047478 (Experiments on long-duration vigilance in dolphins.)