Tired birds sleep more deeply, trading safety for rest

Staying awake when tired feels like walking through fog. For humans, missed sleep dulls our thinking, slows our reflexes, and clouds our focus. Yet we often push through. In the wild, though, the price of sleep deprivation can mean life or death.

Animals have evolved strange and clever sleep strategies to manage this. Birds, in particular, display some of the most fascinating adaptations in the animal kingdom.

One of their most intriguing tricks is the ability to sleep with one eye open. This behavior lets birds keep half their brain alert while the other half rests deeply. It’s a survival skill. But what happens when even this isn’t enough?

A new study on European jackdaws reveals the limits of this behavior. It shows that, when pushed too far, even birds that sleep with one eye open may give in to full, vulnerable rest.

The art of asymmetric sleep

Sleep takes many forms across species. In humans, we move through stages like REM and non-REM sleep in 90-minute cycles. Our brains rest in waves, switching between deep rest and vivid dreams.

Birds follow a similar pattern but in shorter segments. They rest in bursts, adjusting their rhythm to match their surroundings.

Some birds go even further by sleeping asymmetrically. This means one half of the brain stays awake while the other half slips into non-REM sleep.

Ducks often use this trick to remain watchful while still getting rest. The open eye faces out, keeping a lookout for danger. The closed eye faces inward, allowing that side of the brain to recharge.

This type of sleep helps birds navigate dangerous environments. It allows them to remain partially conscious without fully sacrificing rest. But this ability isn’t as powerful as full, deep sleep. It provides protection but not full restoration.

Why deep sleep matters for birds

Asymmetric sleep helps with safety, but it’s not always enough. Deep non-REM sleep clears waste from the brain and helps consolidate memory. It recharges the system.

Doing this with only one hemisphere takes longer and isn’t as effective. That’s why birds still need periods of symmetrical sleep, when both brain hemispheres rest at the same time.

Researchers have long wondered how birds decide when to use one sleep type over another. What triggers the shift? When does safety win, and when does the need to recover push past the risk?

The new study on jackdaws brings these questions closer to answers. By observing how these birds recover from sleep loss, scientists uncovered a tipping point. When deprived of sleep, the jackdaws gave up their asymmetric sleep patterns.

They chose full, deep sleep in both hemispheres, even though it made them more vulnerable to threats.

Exhausted birds choose deep sleep

The research, led by scientists at the Max Planck Institute for Biological Intelligence and the Groningen Institute for Evolutionary Life Sciences, focused on how jackdaws balance safety and sleep. Other studies have shown that many animals use flexible sleep strategies in tough conditions.

Great frigatebirds sleep mid-flight using one side of the brain. Mallard ducks sleep with one eye open when they’re at the edge of their group. Northern fur seals also sleep asymmetrically when floating in the ocean.

Humans aren’t so different. On the first night in a new place, one half of our brain often stays more alert than the other. It’s a kind of natural night watch. But unlike birds, we don’t have the ability to fully control this process or keep one eye open at will.

In jackdaws, sleep deprivation shifted the balance. The birds spent more time in deep, symmetrical sleep, especially early in the night. They needed to recover fast, and asymmetric sleep wasn’t enough.

“Sleep is a dangerous part of life for many animals, as it leaves them exposed to all kinds of risks,” noted Niels Rattenborg, group leader at the Max Planck Institute for Biological Intelligence.

“Some birds can function surprisingly well on far less sleep than we can, but even that strategy appears to have its limits. We have found that when they’re tired, jackdaws are more likely to sleep deeply – even if that means becoming more vulnerable.”

“Observing how birds navigate the trade-off between vigilance and sleep may help us better understand sleep in general.”

Inside the jackdaw brain

To explore how the birds managed their sleep after deprivation, researchers tracked nine jackdaws using EEG recordings. These scans measured brain activity during different sleep phases.

The team wanted to know which parts of the brain rested more deeply, and how that might relate to the bird’s behavior when awake.

Their findings were revealing. Some areas of the brain slept more deeply than others, suggesting that parts that had been more active during the day needed more rest at night. For example, areas linked to vision and decision-making showed stronger sleep signals. The hippocampus, associated with memory, showed less change.

This selective rest could help birds recover without completely losing awareness. But when exhaustion built up, even this wasn’t enough. Deep, full-brain sleep took over. The birds prioritized recovery over safety.

Sleep strategies are shaped by evolution

“Our findings reveal that even the highly flexible sleep strategies used by jackdaws have limits,” said Peter Meerlo, a group leader at the Groningen Institute for Evolutionary Life Sciences.

“That tells us something fundamental about sleep: it’s not just a passive state, but a behavior shaped by both evolution and environmental demands.”

“The study offers a fresh perspective on how animals balance the need for sleep with the risk of predation or other threats – and could help us better understand how sleep loss affects brain function more broadly, including in humans.”

Sleep isn’t just a biological requirement. It’s a behavior shaped by millions of years of challenge and change. Animals like jackdaws show us the trade-offs involved in survival. They sleep lightly when they can, but give in when they must.

What humans can learn from jackdaws

This research on jackdaws goes beyond bird behavior. It speaks to something shared across species. The need to rest. The limits of endurance. And the invisible costs of staying awake too long.

Even the most flexible sleepers hit a wall. The jackdaws remind us that sleep is not optional. Whether in the trees or in our beds, recovery matters. Our brains, like theirs, have limits – and sometimes, the only way to recover is to truly switch off.

The study is published in the journal Current Biology.

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