Sleep is an important process for all animals to conserve energy, rest and heal. Sleep patterns differ across the animal kingdom, depending on where an animal lives and its lifestyle. Animals juggle sleep requirements alongside feeding, breeding and escaping predators. Marine mammals, such as northern elephant seals, must strike this balance alongside the added complication of making time to breathe at the ocean surface.
Northern elephant seals search for food at sea for up to 7 months at a time. They dive deep underwater to feed on small fish that thrive between 200-1000 meters (about 650 to 3300 feet) below the ocean surface. Between these 10 to 30 minute dives, the seals must return to the ocean surface to breathe. However, they limit surface visits to as little as 2 minutes, to avoid predators like killer whales. Scientists don’t know how northern elephant seals find time to sleep, between surface visits, avoiding predators, and diving for food.
A study led by researchers in California used a new device to track sleep in elephant seals. The scientists combined a sensor that takes records of brain activity, called electroencephalograms, or EEGs, with another sensor that takes heart rate records, called electrocardiograms, or ECGs. They paired the sensors with detectors that track dive depth and body movement to create a wearable device for the seals.
The researchers attached the devices to 13 female elephant seals, which typically hunt in the ocean more often than males. They divided the seals into 2 groups, with 5 seals kept in a man-made pool and beach, and 8 released into different locations in the wild. These included a natural beach, shallow water of less than 5 meters (about 16 feet) depth, a continental shelf of less than 200 meters (about 650 feet) depth, and the open ocean, with a depth of more than 200 meters. The researchers retrieved the devices from the seals after 2.5 to 5 days.
By pairing the EEG and ECG data with the body movement data, the researchers noticed that during some dives, the seals in the ocean would begin to glide, showing lower heart activity and changes in brain activity. They saw the seals’ brain waves had lower frequency and higher amplitude during these glides, which they identified as slow-wave sleep, also called deep sleep in humans.
After several minutes, scientists saw the seals stopped all movement and rolled onto their backs. The researchers found the seals’ brain waves then had increased frequency and greater heart activity, which they identified as rapid-eye movement, or REM sleep. During the REM period, the body movement and dive depth data showed the seals circling downwards deeper into the sea in a corkscrew shape, at over 300 meters (nearly 1000 feet) depth. The scientists noticed in shallower areas like the continental shelf, the seals would reach the ocean floor and remain motionless during REM sleep instead.
The researchers identified a total of 104 sleeping dives in the seals’ activity, with each sleeping period lasting under 20 minutes, for a total of around 2 hours sleep over 24 hours. They also noticed the seals on land slept almost 5 times longer in 24 hours than those at sea. The scientists hypothesized their pattern of spiraling deeper into the ocean to sleep could protect the seals from predator attack at the ocean surface while they are sleeping and vulnerable. Meanwhile, the scientists proposed the seals on land had little threat from predators, meaning they could sleep for longer without spiraling.
The scientists wanted to explore their observations using data on a greater number of seals. They used their data to create a mathematical model linking body movement, dive depth, and sleeping patterns in the seals. With this model they could predict seal sleep patterns from their body movements and dive depth. The scientists used the model to analyze diving pattern data they collected on 334 elephant seals between 2004-2019, and identified over half a million sleeping dives in the dataset.
By analyzing their sleeping dives, the researchers confirmed the seals averaged around 2 hours of sleep in 24 hours while at sea. They explained this amount of sleep is comparable to the African elephant, the current record holder for least sleep in mammals.
While the researchers gained new insight into how elephant seals sleep in the ocean, questions still remain. For instance, they don’t yet understand why the seals turn onto their backs while spiraling downwards during REM sleep. The scientists suggested this positioning could be a form of sleep paralysis, but recommended further observation of sleeping seals to test this hypothesis.