Dolphins rest one cerebral hemisphere at a time in a sleep state called unihemispheric slow‑wave sleep, allowing them to sleep while remaining partly alert
Unihemispheric slow‑wave sleep
Dolphins enter unihemispheric slow‑wave sleep in which one hemisphere shows sleep‑like slow waves while the other remains in a wakeful state, enabling rest without full loss of consciousness.
Conscious breathing and surfacing
Because dolphins must breathe consciously rather than relying on an automatic respiratory reflex, keeping one hemisphere active ensures they can surface for air and control breathing while the other side rests.
Vigilance and predator awareness
The hemispheric sleep arrangement preserves vigilance so dolphins can monitor for threats, maintain slow locomotion near the surface, and coordinate surfacing and respiration even during extended periods of rest.
Behavioural and experimental evidence
Behavioural studies and experimental work show dolphins can perform sensory tasks and echolocate effectively while undergoing prolonged cycles of unihemispheric sleep, demonstrating sustained alertness in the awake hemisphere during extended monitoring periods.
Variations across species and contexts
Patterns of unihemispheric sleep vary by species, age and ecological context: some cetaceans adopt slow swims near the surface, others rest motionless at the surface, and mother‑calf pairs use staggered sleep to maintain neonatal care and safety.
Takeaway
By sleeping with half their brain at a time, dolphins balance physiological need for rest with the demands of conscious breathing and environmental vigilance, a specialised adaptation that supports survival in an aquatic, air‑breathing lifestyle.