Unihemispheric Sleep: How Marine Mammals Rest and Stay Alert in Open Water
Marine mammals have a unique way of getting their much-needed rest while navigating the open waters. Unlike humans, who can simply close their eyes and drift off to sleep, marine mammals face the challenge of needing to surface for air regularly. Additionally, sleeping at the water’s surface leaves them vulnerable to predators and heat loss. So how do these fascinating creatures manage to get some shut-eye without putting themselves at risk? The answer lies in a phenomenon known as unihemispheric sleep.
Unihemispheric sleep is a sleep pattern where marine mammals shut down one half of their brain at a time. This allows them to maintain a low level of activity while still getting some rest. Dolphins, in particular, are the best-studied marine mammals capable of this style of sleeping. Brain scans on captive dolphins have revealed that while one hemisphere is in deep sleep, the other hemisphere remains alert, allowing the animals to sleep with one eye open, quite literally.
Patrick Miller, a biologist at the University of St Andrews in the U.K., explains the significance of unihemispheric sleep for these animals, stating, “Unihemispheric sleep is really valuable to these animals because it allows them to maintain a low level of activity while still sleeping half of their brain at a time.” This unique sleeping pattern is not exclusive to dolphins but is also common among cetaceans, which include dolphins, whales, and porpoises. Interestingly, many bird species also utilize unihemispheric sleep, often allowing them to doze off while flying.
While birds and dolphins both employ the half-brain sleeping technique, they do so for different purposes. In a flock of birds, those on the outside keep the eye facing away from the group open to watch for predators. However, dolphins do the opposite. While sleeping, they tend to keep the eye open that faces the rest of the pod, likely to avoid becoming separated.
Not all cetaceans exhibit unihemispheric sleep. Some, like humans and most other mammals, experience bihemispheric sleep, where both hemispheres of the brain fall asleep simultaneously. However, studying the sleep patterns of animals out at sea can be challenging. For instance, it is difficult to measure brain activities for animals like sperm whales, blue whales, or humpback whales that cannot be captured. In such cases, behavioral records become the best indicator of sleep behavior.
Researchers have found ways to monitor the sleep behavior of these elusive creatures. In a 2008 study led by Patrick Miller, suction tags were attached to sperm whales to observe their behavior. The study showed that sperm whales sleep out in the open ocean for short bouts. They make shallow dives beneath the surface, slow their swim speed to a stop, and pitch lazily upward. This upward pitch is likely due to the buoyant oil, known as spermaceti, in their heads.
During sleep, entire pods of whales can be seen nose-up just below the ocean’s surface. They appear completely unresponsive, indicating a form of deep sleep. However, they can only rest underwater for about 20 minutes before needing to resurface for air. Once they catch their breath, they descend below the surface for more rest and may continue this behavior for up to 3.5 hours.
Northern elephant seals also exhibit bihemispheric sleep and engage in similar short bouts of sleep. In a groundbreaking 2023 study led by Jessica Kendall-Bar, brain activity in a sleeping mammal at sea was monitored for the first time. The study found that elephant seals dive to a depth of around 1,000 feet before their brains slow down and enter rapid eye movement (REM) sleep. During REM sleep, the seals flip upside down and spin in a slow circle while continuing to sleep.
Kendall-Bar explains this peculiar behavior, stating, “Seals sleeping in the ocean were upside down 100% of REM sleep time, suggesting that just like us, they become paralyzed during REM.” However, due to the risk of predation, elephant seals limit their total sleep time at sea to only around two hours per day. This rivals the record for the least sleep among all mammals, currently held by the African elephant.
The study of unihemispheric sleep in marine mammals provides valuable insights into their unique adaptations for survival in open water. By shutting down one half of their brain at a time, these remarkable creatures can rest while remaining alert to potential dangers. Whether it’s dolphins keeping an eye on their pod or whales finding moments of respite beneath the waves, unihemispheric sleep is a fascinating phenomenon that highlights the incredible adaptability of marine mammals.
