During rumination, the re-chewing of partially digested food, the brainwaves of reindeer resemble the brain waves present during non-REM sleep. According to a new study, this may help them to get enough sleep in the summer, in preparation for the harsh winter conditions.

Reindeer (Rangifer tarandus) are native to the Arctic, where light-dark cycles are absent during summer and winter. Previous studies showed that reindeer don’t display circadian behavioral rhythms during these seasons, though they seem more active during daytime in the spring and autumn equinox, when there are light-dark cycles.

However, it was not clear whether these seasonal differences affect how much, and how well, reindeer sleep. To investigate this further, researchers from the University of Zurich used non-invasive electroencephalography (EEG) to monitor Eurasian tundra reindeer (Rangifer tarandus tarandus) during the autumn equinox and the summer and winter solstices.

They found that reindeer slept for the same amount of time during winter, summer and autumn, despite being more active during summer. This contrasts with other species who change how much they sleep in response to environmental conditions. On average, reindeer spent 5.4 hours in non-REM sleep, 0.9 in REM sleep and 2.9 hours ruminating during a 24-hour period, regardless of the season.

“The fact that reindeer sleep the same amount during winter and summer implies that they must have other strategies to cope with limited sleep time during the arctic summer,” first author and neuroscientist Melanie Furrer, said in a news release.

One possible strategy is resting during rumination, an important component of digestion for reindeer and other ruminants. Sheep, goats, cattle and Lesser mouse-deer have been previously observed producing sleep-like brain waves during rumination, but it wasn’t clear whether rumination could have a restorative function like sleep does.

Asleep on their feet

The researchers found that the reindeer’s EEG readings during rumination were similar to brainwave patterns that are indicative of non-REM sleep, such as increased slow-wave activity. Sleeping and ruminating reindeer also showed similar behavior, tending to sit down or stand during both activities, and were less reactive to disturbances, such as a nearby reindeer sitting down or getting up.

They also tested whether rumination could reduce the reindeer’s drive to sleep by depriving the reindeer of sleep for two hours and measuring the brain waves during and after. The EEG readings after sleep deprivation showed increased slow-wave activity, which indicates a build-up of sleep pressure — the biological drive for more and deeper sleep.

However, after ruminating, the slow-wave activity of reindeer was decreased during subsequent sleep. And the more they ruminated, the more the slow-wave activity decreased. “This suggests that rumination reduces sleep pressure, which could benefit the reindeer because it means they don’t have to compromise on sleep recovery when they spend more time ruminating,” Furrer said.

This is especially relevant during the summer, as they more they eat, the more time the reindeer need to spend ruminating. “Rumination increases nutrient absorption, so it’s crucial for reindeer to spend enough time ruminating during the summer in order to gain weight in anticipation of winter,” said Furrer.

The researchers called for follow-up studies to compare the impact of ruminating while sleeping with rumination while awake. They would also like to measure reindeer behavior and sleep in more natural outdoor conditions, considering their study was done in indoor stables. However, they acknowledge that this would require implanted EEG sensors instead of the non-invasive electrodes they used.

The study was published in the journal Current Biology.