How do seals sleep underwater?

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How do they sleep?

Seals spend roughly a third of their life on dry land and two thirds in the sea. They sleep in both places — and even underwater. Their sleep is not like that of a human or a dog: seals doze in short cycles of 5 to 20 minutes, with breathing pauses at the surface in between. An adult seal sleeps about 6 to 10 hours per 24-hour period, often spread over dozens of dozing phases.

1. Floating at the surface ("bottling")

The best-known sleeping pose is bottling: the seal hangs vertically in the water with only the nose above the surface, like a bottle sticking out of a tub. Grey seals in sheltered bays especially do this. It's efficient rest: breathing requires no effort, and the thick blubber layer keeps the animal afloat without muscle activity. Sometimes you see an animal drifting on its back, with its belly up.

2. On the seabed in shallow water

In shallow waters (up to 30 metres) a seal can simply lie on the seabed, curled up, and sleep there. Every few minutes it swims up in a few strokes, breathes, and sinks back down. To the observer at the surface the animal seems to have vanished — but it is sleeping calmly four or five metres beneath your feet. This behaviour is seen especially in common seals in the Waddenzee.

3. On dry land

The deepest, longest sleep takes place on an exposed sandbar. There seals lie still for hours, sometimes on their back. Research on true seals shows they can reach deep sleep and REM sleep both on land and underwater. Quiet sandbars still matter: that is where they rest, digest, warm up and nurse their pups, so disturbing resting sites costs them energy and rest.

Sleeping on a held breath

Humans breathe automatically. Our brainstem responds to rising CO₂ with a strong urge to inhale. Seals exhale before they dive and then hold their breath, even while asleep. They tolerate much higher CO₂ levels than we do and carry large oxygen stores in blood and muscle, so they can doze underwater for minutes and surface by themselves to breathe. If they get entangled in a net they cannot surface, and eventually drown.

The short doze cycle

Scientists who have studied seals in research tanks describe a typical sleep cycle as follows:

1. Deep breathing at the surface (3–5 breaths).
2. Sinking under water, sometimes to the bottom.
3. 5–15 minute dozing phase with strongly reduced heart rate (from 100 to 10 beats per minute).
4. Automatic waking and surfacing in a few strokes.
5. Repeat.

The physiological tricks that make this possible — high myoglobin in muscles, slowed heart rate, redistribution of blood to the core — are the same that seals deploy during ordinary diving. Read more in how deep do seals dive.

Difference from dolphins

Dolphins show unihemispheric sleep: one brain half sleeps at a time while the other stays alert. In true seals, such as our common and grey seals, it is different: brain recordings show that both hemispheres sleep at the same time, even underwater while they hold their breath. Eared seals (sea lions and fur seals) differ again: in water they often sleep with one brain half, like dolphins.

Common seal pups swim within an hour of birth, and during their first weeks learn independently how and where to rest. A washed-up, emaciated pup often suffers sleep deprivation from disturbance — an argument for leaving sandbars alone. Find a weakened seal? Follow our seal pup protocol.