Dolphins, as marine mammals, have developed an extraordinary method of rest that reflects their unique aquatic environment. Unlike most creatures that can fully succumb to sleep, dolphins face challenges like the constant need to surface for air and the presence of threats. This necessitates a specialized evolutionary solution, allowing them to rest while remaining aware of their surroundings.
How Dolphins Sleep
Dolphins employ a remarkable sleep pattern known as Unihemispheric Slow-Wave Sleep (USWS). In this state, only one half of their brain enters a deep sleep, while the other hemisphere remains awake and functional. This allows them to continue essential activities even while resting. A noticeable sign of this unique sleep is that dolphins often swim with one eye closed; the closed eye corresponds to the resting half of the brain.
The awake half of the brain monitors the environment and controls bodily functions. Dolphins periodically switch which side of their brain is resting, ensuring both hemispheres get adequate sleep. Each hemisphere typically gets about four hours of slow-wave sleep, totaling approximately eight hours of rest per day. This alternating pattern ensures they can continuously move and remain alert in their aquatic habitat.
Why Dolphins Sleep This Way
The specialized sleep pattern of dolphins is directly linked to their survival. Dolphins are conscious breathers, meaning they must actively decide to surface for air, unlike humans whose breathing is involuntary. If a dolphin were to enter complete unconsciousness, it would stop breathing and likely drown. USWS enables them to maintain conscious control over their breathing, ensuring they regularly surface through their blowhole for oxygen.
Remaining partially awake also allows dolphins to stay vigilant against predators, such as sharks. This helps them react quickly to danger. Additionally, keeping one brain hemisphere active allows them to maintain muscle movement, which helps regulate their body temperature in the often cold ocean waters. This unique sleep adaptation facilitates both safety and physiological stability.
How Baby Dolphins Sleep
Newborn dolphins exhibit a distinct sleep pattern, characterized by a significant reduction in rest during their initial weeks of life. Calves have less blubber than adults, making it harder for them to float without constant movement. Consequently, they must swim almost continuously to maintain their body temperature and to surface for air. This period of high activity typically lasts for at least the first two months.
The mother dolphin plays a significant role in her calf’s early development by engaging in “echelon swimming”. In this formation, the calf swims closely alongside its mother, positioning itself within her slipstream. This reduces the effort required for the baby to keep pace and surface for breaths. Both mother and calf show high levels of activity and vigilance during this period, with the calf’s eye often remaining open towards the mother.