Their entire sleeping pattern was fragmented into thousands upon thousands of tiny episodes.
The researchers found that nesting chinstrap penguins could enter sleep more than 600 times per hour.
Across an entire day, that could mean more than 10,000 separate microsleeps.
The average episode of slow-wave sleep lasted only about four seconds. Most sleep bouts lasted less than 10 seconds, although longer episodes occasionally occurred. ABC Science reported that the longest recorded bout in the study reached 34 seconds.
Four seconds hardly sounds like sleep at all.
Yet when these fragments were added together, they amounted to a substantial total.
The researchers calculated that the penguins accumulated more than 11 hours of sleep per hemisphere each day.
In other words, they were not necessarily sleeping less.
They were sleeping differently.
Thousands of Tiny Pieces of Rest
That distinction is important.
When people hear that an animal sleeps for only four seconds at a time, it is easy to imagine that the animal must be severely sleep-deprived.
The chinstrap penguins complicate that assumption.
Their sleep was extraordinarily fragmented, but there was a huge amount of it.
Instead of getting several long blocks of rest, they collected sleep in tiny installments throughout the day and night.
A few seconds here.
Another few seconds there.
Repeated hundreds of times every hour.
Eventually, those fragments accumulated into hours of neurological sleep.
The study therefore raised a fascinating question about one of biology’s most familiar behaviors: Does sleep always need to be continuous to provide benefits?
Microsleeps are well known in humans, but they are generally associated with fatigue rather than healthy rest. A tired person might briefly nod off while sitting at a desk or watching television. More dangerously, microsleeps can occur when someone is driving or operating machinery.
Because human microsleeps are so short and frequently occur when someone is sleep-deprived, researchers have traditionally questioned whether such brief episodes can provide meaningful restorative benefits.
Chinstrap penguins suggest that the answer may depend heavily on the species and circumstances.
The original researchers noted that the successful breeding of penguins using this strategy suggests that at least some benefits of sleep may accumulate incrementally through repeated short bouts. Importantly, however, the study did not directly measure every possible restorative effect of microsleep, so it would be too strong to claim that scientists have proved four-second naps are equivalent to normal continuous sleep in every respect.
What the findings do demonstrate is that these penguins can maintain an astonishingly fragmented sleep schedule while continuing to function and reproduce successfully.
Sleeping With Half the Brain
The story becomes even more unusual.
The researchers found that chinstrap penguins experienced both bihemispheric and unihemispheric slow-wave sleep.
Bihemispheric sleep means sleep-related slow-wave activity occurs in both hemispheres of the brain.
Unihemispheric sleep, by contrast, involves one cerebral hemisphere sleeping more deeply while the other remains more alert.
This ability is found in several groups of animals and is particularly useful in situations where complete disengagement from the environment could be dangerous.
Some marine mammals use unihemispheric sleep because they need to continue surfacing to breathe. Various birds can also use it under circumstances that demand vigilance.
In the chinstrap penguins, the combination of extremely brief sleep bouts and the ability to sleep with one hemisphere at a time adds another layer to their unusual strategy.