1. Wild Sloths Slept 6 Hours Less Than Captive Sloths
Sloths are animals that move very little, so people have a strong image of them as sleepy creatures. It has long been reported that sloths sleep for about 16 hours a day in captivity [2].
In a study on Barro Colorado Island in Panama, researchers attached small brainwave monitors to three female three-toed sloths living in the wild. The results showed their daily sleep was about 9.6 hours. This was about 6 hours shorter than studies of captive sloths. It is said to be the first time wild animal sleep was measured using brainwaves [1].
The reason for this difference is not clear. Possible explanations include the presence of young individuals in the captive group, and the fact that wild sloths may sleep less because they are searching for food or staying alert to danger [2].
It is important to note that data was recorded from only three animals over a period of 3 to 5 days [2]. This result alone does not determine the sleep time for all wild sloths.
We were able to measure sloth sleep using brainwave monitors. But how do we measure the sleep of elephants, which are large and difficult to approach in the wild?
2. How Elephant Sleep Was Measured: Trunk Not Moving for 5 Minutes
In a study at Chobe National Park in Botswana, researchers observed two wild African elephants for 35 days. These were female herd leaders. Their average daily sleep was about 2.0 hours. This was reported as the shortest recorded for any mammal [3].
The method of measurement is crucial here. Implanting brainwave electrodes in wild elephants would place a heavy burden on their bodies. Instead, researchers implanted activity monitors in the noses and defined sleep as "the nose not moving for more than 5 minutes." They used a substitute indicator rather than direct brainwaves. The authors themselves stated that this method might count quiet waking time as sleep, potentially leading to higher estimates [3].
Comparative numbers exist. Captive elephants are recorded as sleeping for about 3 to nearly 7 hours. The two wild elephants slept 1 to 4 hours less than this. In the wild, there were days when they did not sleep at all for up to 46 hours [4]. Some suggest predators or poachers might influence this, but these causes are speculative.
Because this result comes from only two animals, it is not an average for all elephants [3].
The elephants did not appear to "bank" sleep after short nights. However, the observed time alone does not reveal how much sleep elephants actually need. There is a tendency for larger animals to sleep less, but why this happens is still under debate [4].
We have seen that wild conditions often lead to shorter sleep. Are there extreme examples where "where" an animal is changes its sleep?
3. Great Frigatebirds Flying Over the Sea Sleep Only 42 Minutes a Day
The great frigatebird is a bird that can fly over the sea for up to about 10 days at a time. When researchers attached brainwave monitors to females in the Galapagos, they found that sleep during flight averaged about 42 minutes per day. On land, these birds slept for more than 12 hours [5].
42 minutes is less than one-seventeenth of the 12+ hours on land (the actual ratio is even smaller because land sleep was "more than" 12 hours). This shows that for the same bird and body, sleep time changes greatly depending on location.
During flight, sometimes half the brain slept while the other half stayed awake. At other times, both sides of the brain slept simultaneously [5].
Why the sleep is so short is not known. The authors of the paper suggest that sleep during flight might be insufficient [6]. Whether it is enough or not cannot be decided by numbers alone.
[SAFETY NOTE] During flight, sleep can take the form of unihemispheric sleep (where only one brain hemisphere sleeps) or both hemispheres sleeping at once [5]. Recognizing that both types exist is important when considering how animals sleep safely while flying. It is safe to read the 42 minutes as "the amount observed during flight," not necessarily "the amount the bird needs."
Marine animals also show changes in sleep based on location.
4. Northern Fur Seals Rarely Have REM Sleep While in the Water
The northern fur seal lives both on land and in the sea. A study of four young captive seals found that on land, they had an average of about 80 minutes of REM sleep per day [7]. REM sleep is said to be a type of sleep where dreaming is likely.
However, in seawater, for periods of up to about two weeks, very little REM sleep was seen (less than 3 minutes). Even when they returned to land, the amount recovered was small [7]. Underwater, their sleep switched to a pattern where one side of the brain slept at a time [7].
When we line up these examples, we see that both the amount and the type of sleep change based on "where" the animal is: wild vs. captive, land vs. air vs. sea.
So, when comparing numbers, what exactly are we counting as "sleep"?
5. Why Sleep Numbers Change Depending on How Sleep Is Defined
Animal sleep is generally understood through conditions such as "not moving," "greatly reduced reaction to stimuli," "able to return to wakefulness quickly," and "stronger sleepiness if deprived" [8].
However, some animals are hard to classify with these conditions alone. For example, sharks continue swimming, and frogs remain alert even when resting. This explanation comes from a personal website, so definitions should be read as rough guides [8].
Measurement methods also differ. Sleep time changes with the environment; for instance, hunger can reduce it. Many studies observe animal behavior without using brainwaves [8].
If the "ruler" used is different—nose movement for elephants, brainwaves for sloths—the meaning of the numbers differs too. Therefore, charts of sleep times by species often mix studies with different measurement methods.
When we see such a chart, what should we look for?
6. How to Read Sleep Time Charts: Check Where and How Animals Were Measured
Commonly cited charts list numbers like: brown bats at 19.9 hours, sloths at 14.4 hours, cats at 12.1 hours, dogs at 10.6 hours, giraffes at 4.5 hours, horses at 2.9 hours, and wild African elephants at 2.1 hours [9].
The sloth figure of 14.4 hours does not match the 16 hours or 9.6 hours seen in this article. This is because charts may mix numbers from captive and wild settings, using different measurement methods. The elephant's 2.1 hours is from a different source than the approx. 2.0 hours mentioned earlier. There is also a note that bat figures might include torpor (dormancy), but this chart does not explain its measurement method [9].
So, when looking at a chart, there are two things to search for: "Where was it measured?" and "What was used to measure it?" If you cannot find this information, it is safe to treat the number as a rough estimate.
Try it out. If you have a cat or dog at home, use a clock to measure the time they lie still and do not move. You will notice that even resting with eyes open counts as "not moving." This lets you experience the same challenge faced by the elephant activity monitor researchers. If you do not have pets, look in encyclopedias or books for sleep charts and check if the "measurement method" is listed. Even if it is not, you can think about it using the elephant example from this text.