Unihemispheric Sleep: EEG Insights from Resting Dolphins at Dolphin Academy Curaçao

When do dolphins sleep if the ocean never stops moving? At Dolphin Academy Curaçao, researchers use electroencephalogram (EEG) measurements to study unihemispheric sleep—a remarkable pattern where one hemisphere of the brain rests while the other stays awake. In this post, you’ll learn what unihemispheric sleep is, how EEG helps reveal resting brain activity, what long-term observations in Curaçao are uncovering, and why these findings matter for dolphin care and education.

What is unihemispheric sleep in dolphins?

Definition (concise): Unihemispheric sleep is a rest state in which dolphins let one brain hemisphere sleep while the other remains awake and active.

• Past studies have shown dolphins can rest one hemisphere at a time.
• This pattern allows ongoing awareness and movement even during rest.
• Dolphin Academy has reproduced EEG testing in resting dolphins to build a baseline for its dolphin population and support a multi-year research program.

How EEG helps us see a resting dolphin’s brain at work

Electroencephalogram (EEG) records electrical activity from the brain. In resting dolphins, EEG measurements provide a direct window into the neural patterns associated with unihemispheric sleep.

• At Dolphin Academy, EEG measurements are conducted in resting dolphins to establish a baseline dataset for the resident population.
• This baseline enables systematic study of:
  • Rest/wake cycles from birth to maturity
  • Sensory responsiveness of resting dolphins (arousal threshold)
  • Time–place association in dolphins
  • Hormone sampling and measurement using saliva and blow-hole chuffs

Why this matters: By pairing brain activity with behavior and physiology, the team can relate when and how dolphins rest to where they rest, who they rest with, and how they respond to their environment.

A multi-year look at daily rhythms in Curaçao

Beginning in January 2013, a “sleep-like resting behavior” project began, built on non-invasive, around-the-clock observations.

• Each year, at the end of the rainy season (end of February, beginning of March), researchers observe dolphins during multiple time periods.
• Over time, this cadence produces a full 24/7 picture of daily patterns for the community.

What’s emerging from these observations:

• A clearer view of how much time each dolphin prefers to sleep
• Preferred resting locations
• Preferred associates during rest and activity

The growing dataset spans life stages and social contexts:

• How time is allocated in early life
• Days and nights during the transition into semi-independent calves
• Expanding activity ranges as calves grow to adolescence
• Factors shaping daily rhythms in adulthood

As data accumulate, researchers can look for:

• Trends in rhythmic variation across years
• Differences between males and females
• How being a full-time mother influences rest/wake patterns

Mother–calf rest and movement: what we’re seeing

A central focus of the long-term sleep study is the development of rest/wake behavior in pre- and post-partum periods for mothers and their newborn calves.

• Dolphins are among the few mammals that remain in constant movement after birth.
• Calves tend to sleep very little in the first days and even weeks; it is even questionable if they “sleep” at all early on.
• Unlike most mammals (including humans), dolphin sleep increases with age.

Implication: Tracking these early-life patterns helps build age-specific baselines and informs how rest evolves from birth through maturity.

Do resting dolphins "tune out"? Testing sensory responsiveness

A key open question for resting bottlenose dolphins is whether they exhibit an increased arousal threshold—that is, decreased sensory responsiveness at deeper stages of rest.

• In humans, lighter sleep is easily disrupted; deeper sleep resists awakening.
• Because dolphins always keep part of the brain awake, do they maintain similar responsiveness while resting?

To investigate, the team is in the last stages of testing arousal thresholds by broadcasting different frequencies and volumes of sounds through an underwater speaker while dolphins exhibit resting behavior.

Why this matters: If responsiveness changes with depth of rest, it illuminates how dolphins balance safety and restorative rest in dynamic environments.

Beyond brain waves: integrating hormones, place, and time

EEG is part of a broader, corroborative research effort at Dolphin Academy to understand how behavior, biology, and context fit together.

• The baseline dataset supports:
  • Rest/wake cycle tracking from birth to maturity
  • Time–place association in daily routines
  • Hormone sampling using saliva and blow-hole chuffs

By aligning brain activity, behavior, location, social partners, and physiological signals, researchers can map how rest patterns relate to daily life in the lagoon and beyond.

Key terms (quick reference)

• Unihemispheric sleep: One brain hemisphere sleeps while the other stays awake.
• EEG (electroencephalogram): A method for measuring brain activity.
• Arousal threshold: How easily a resting animal responds to stimuli; higher thresholds mean deeper rest.
• Blow-hole chuffs: Exhalations at the blow-hole that can be sampled for hormone measurements.

Practical takeaways for students, educators, and visitors

• If you’re studying marine biology or animal behavior:

  • Use the unihemispheric sleep framework to think about how rest supports vigilance, navigation, and social behavior in aquatic mammals.
  • Compare rest/wake development across life stages to understand how calves transition into independent sleepers.
  • Consider arousal thresholds as a functional measure of sleep depth in species that remain partially alert.

• If you’re an educator or group leader:

  • Connect classroom lessons on sleep and circadian rhythms with real-world examples from Dolphin Academy’s research.
  • Encourage students to define key terms (EEG, arousal threshold) and pose testable questions about behavior and environment.

• If you’re planning a visit to Curaçao:

  • Explore related pages such as Dolphin Research, Dolphin Health Care, and Open Sea Training to deepen your understanding.
  • Pair learning with memorable Experiences like the Dolphin Encounter, Dolphin Swim, Dolphin Snorkel, Dolphin Scuba Encounter, or Open Water Dolphin Dive.
  • For youth and school groups, check out Youth Activities and the Assistant Trainer Course for hands-on, educational context.

FAQs about unihemispheric sleep and EEG

Do dolphins really sleep with half their brain?

Yes. Dolphins exhibit unihemispheric sleep, resting one hemisphere while the other remains awake and active.

How does EEG help study dolphin sleep?

EEG measures brain activity in resting dolphins, enabling researchers to establish baselines and study how rest/wake cycles, responsiveness, and physiology align over time.

Are resting dolphins less responsive to sounds?

That’s under active investigation. Sensory responsiveness (arousal threshold) is being tested using different sound frequencies and volumes played underwater while dolphins rest.

What is unique about calves and sleep?

Calves often sleep very little in the first days and weeks after birth, and dolphin sleep increases with age—opposite to many mammals.

Conclusion: Mapping rest to daily life in Curaçao

Unihemispheric sleep lets dolphins rest while staying active—an elegant solution visible in EEG recordings from resting animals at Dolphin Academy Curaçao. Combined with non-invasive, around-the-clock observations each year and integrated measures such as hormone sampling, this multi-year research program is building a life-long dataset of daily rhythms across ages, social roles, and environments. The result is a clearer, community-wide picture of when, where, and how dolphins rest—and how that rest supports healthy lives.

Ready to learn more or get involved? Explore Dolphin Research and Dolphin Health Care, discover Open Sea Training, and plan your visit through our Experiences. Educators and aspiring professionals can also check Youth Activities, the Assistant Trainer Course, and Join our team to connect passion with purpose.