Reindeer can chew while they sleeping

For those in the northern hemisphere, December means holiday festivities are in full swing. We rush around decorating, baking, shopping, and hosting parties. With so much to do, it can feel impossible to find time for everything, including sleep. But it turns out even the hardworking reindeer of the Arctic have figured out how to multitask better than we do. New research shows these iconic Christmas creatures have discovered an ingenious way to rest and eat at the same time, and the science behind it is genuinely fascinating.

Credit: Pexel

Quick Facts

• The study was published on 22 December 2023 in the journal Current Biology
• Researchers monitored four female reindeer in Norway using EEG electrodes
• Brainwave patterns during cud-chewing closely resembled those of non-REM sleep
• The more reindeer ruminated, the less additional sleep they needed afterward

The Study, Explained

Scientists from the University of Zurich, working with colleagues at UiT The Arctic University of Norway, set out to understand how reindeer manage their sleep in one of the most extreme light environments on Earth. In the high Arctic, summer brings 24-hour daylight and winter brings near-total darkness, which strips away the normal day-night cues most animals rely on to regulate sleep.

Led by neuroscientist Melanie Furrer, the team attached non-invasive EEG electrodes to four female reindeer to record their brain activity around the clock. What they found surprised even the researchers: when the reindeer were calmly chewing cud, regurgitated plant matter that ruminants like reindeer, cows, and deer re-chew to aid digestion, their brainwaves showed the slow-wave activity and sleep spindles characteristic of non-REM sleep, the deep, restorative stage of sleep that helps the brain and body recover.

How Rumination Mimics Sleep

The reindeer weren’t just physically still while chewing. Behaviorally, they appeared calm and relaxed, often with their eyes closed, and were noticeably harder to startle or distract than reindeer that were fully awake and alert. Rustling or movement from nearby reindeer was less likely to catch a ruminating reindeer’s attention, a pattern consistent with a genuinely reduced state of arousal rather than simple idle chewing.

To test whether this rumination actually substituted for real sleep, the researchers kept the reindeer awake for two hours and then measured their brain activity. Sleep-deprived animals typically show a rebound of intense slow-wave activity once they finally do sleep, the brain’s way of “catching up” on lost rest. The reindeer showed this rebound too, but the more time they had spent ruminating beforehand, the smaller that rebound was. In other words, chewing cud appeared to genuinely reduce their need for additional recovery sleep, not just occupy time.

Why This Matters for Arctic Survival

This multitasking ability looks like a remarkably useful evolutionary adaptation. During the brief Arctic summer, when food is abundant and daylight never ends, reindeer feed almost continuously to build up reserves before the long, food-scarce winter arrives. Every hour spent purely sleeping is an hour not spent eating, and every hour spent purely eating is an hour the body doesn’t get to rest and recover. Being able to do both at once appears to let reindeer maximize foraging time during the short summer window without sacrificing the restorative benefits of sleep.

Reindeer aren’t the only Arctic-adapted creatures without a normal day-night sleep cycle. Earlier research on wild reindeer found that during the constant light of summer and constant darkness of winter, reindeer show little of the regular daily activity rhythm most mammals display, becoming more clearly day-active only around the spring and autumn equinoxes when normal light-dark cycles briefly return. The ability to sleep while ruminating may be one of the tools that lets them function well even when the usual light-based cues for sleep simply aren’t available.

Other Animals With Unusual Sleep Strategies

Reindeer are in good company when it comes to creative sleep solutions. Frigate birds, which can stay airborne for weeks during long ocean crossings, are able to sleep in short bursts while still flying. Some seal species can sleep while diving underwater, a strategy that helps them rest while staying safely away from predators on land. Even some of our own site’s coverage of how animals sleep and whether they dream touches on just how varied sleep strategies are across the animal kingdom, and reindeer’s cud-chewing trick fits right into that pattern of nature finding creative workarounds for survival.

As sleep physiologist Vladyslav Vyazovskiy of the University of Oxford, who wasn’t involved in the study, put it in comments to Smithsonian Magazine, sleep doesn’t have to look the same across every species. The traditional assumption that eating and sleeping are mutually exclusive activities simply doesn’t hold up for every animal, and reindeer are one of the clearest examples of that so far discovered.

Frequently Asked Questions

Do reindeer really sleep while chewing?
Yes. A 2023 study published in Current Biology found that reindeer brainwaves during cud-chewing closely resemble those of non-REM sleep, and that this rumination genuinely reduces how much additional sleep the animals need afterward.

Why do reindeer need to sleep and eat at the same time?
Arctic summers bring 24-hour daylight and abundant food, giving reindeer a narrow window to build up fat reserves before winter. Being able to rest while ruminating lets them maximize feeding time without sacrificing the restorative benefits of sleep.

What is rumination?
Rumination is the process by which animals like reindeer, cows, and deer regurgitate partially digested food (cud) and chew it again to break it down further, which improves digestion of tough, fibrous plant material.

Do other animals sleep in unusual ways too?
Yes. Frigate birds sleep in short bursts while flying during long migrations, and some seals sleep while diving underwater to stay safe from predators, both remarkable adaptations to environments where normal, uninterrupted sleep isn’t practical.

A Fitting Discovery for the Season

There’s something fitting about this discovery landing right around the holidays. Santa’s reindeer are famously tireless, pulling a heavily loaded sleigh across the globe in a single night. Real reindeer, it turns out, may have a genuine biological trick that helps explain how animals built for endurance manage to keep going: the ability to rest and refuel at exactly the same time.

This holiday season, there’s a small lesson worth borrowing from Arctic reindeer. Finding low-effort ways to combine two necessary tasks, rather than treating them as competing demands on your time, might be one of nature’s oldest efficiency hacks.