Seals are marine mammals with unique sleeping habits. A common question is whether seals sleep underwater. These adaptable creatures can sleep in both aquatic and terrestrial environments, employing distinct strategies tailored to their surroundings.
Underwater Slumber
Seals can sleep underwater, though their methods vary by species and situation. Some, like harbor seals, often float vertically in the water, a position called “bottling,” with their noses just above the surface for breathing. They may also drift horizontally at the surface, resembling a log swaying with the waves. This relaxed posture helps them conserve energy while getting short periods of rest.
Other species, particularly elephant seals, take their underwater naps to greater depths. These deep-sea naps typically last around 10 minutes, occurring during dives that can extend for 30 minutes or more. During these “sleep spirals,” seals may lose postural control and drift downwards, sometimes flipping upside down. Sleeping submerged allows seals to rest in areas with lower predation risk, as many predators lurk near the surface.
Physiological Adaptations for Aquatic Rest
Seals’ ability to sleep underwater relies on specialized physiological adaptations. A primary mechanism is the mammalian diving reflex, triggered by submersion in water. This reflex involves a slowing of the heart rate, known as bradycardia, which can reduce a seal’s heart rate during extended dives. Simultaneously, peripheral vasoconstriction occurs, redirecting blood flow away from non-essential tissues towards the heart and brain.
Seals also possess efficient oxygen storage capabilities. Their blood carries more oxygen, and they have large blood volumes. Muscles contain high concentrations of myoglobin, a protein that stores oxygen for prolonged dives. Additionally, their spleen can contract to release a reserve of oxygen-rich red blood cells into circulation. Many seal species exhibit unihemispheric slow-wave sleep (USWS) while in water, where one half of the brain rests while the other remains alert. This allows them to maintain vigilance for predators and surface for air when necessary, ensuring survival during aquatic rest.
Terrestrial vs. Aquatic Sleep
Seals demonstrate flexibility in their sleeping habits, choosing land or water based on various factors. On land, seals achieve deeper sleep, including REM sleep, which is often limited or absent during aquatic rest. Land-based sleep offers advantages such as complete muscle relaxation and no need to interrupt rest for breathing. Seals frequently haul out onto beaches, rocks, or ice to rest, sometimes congregating in large groups for warmth and increased safety from land-based threats.
While terrestrial sleep offers deeper rest, aquatic sleep provides safety from land predators and aids in thermoregulation. Underwater sleep is shallower and interrupted by the need to surface for air. For instance, elephant seals average about two hours of sleep per day when at sea, but can sleep for up to 14 hours daily when on land. The choice between land and water for sleep depends on the species, predator presence, and specific activities like molting or pupping.