Have
you ever wondered whether your dog sleeps like you – and yaps because
he is probably dreaming about the mailman? Do all animals need sleep as
much as we do? And how on Earth do they continue swimming or even flying
during sleep?
Smart because of Mattresses?
First
of all, we should define what we are talking about: Sleep can be
characterized in many ways, but without electroencephalography with wild
animals, we should focus on the behavioral definition: Sleep is when
the animal exhibits a rapidly reversible state of immobility and reduced
responsiveness to external stimuli. Furthermore, an increased drive for
sleep (rebound effect) is expected after sleep deprivation [1].
Lets
start with our closest relatives: Great apes exhibit monophasic sleep
(like humans and unlike the majority of other mammals), meaning that
they concentrate their sleep in one period per day. Theories claim the
reason for this is that they are (like us) capable of building a
comfortable and safe sleeping platform - allowing them to sleep more
safely and therefore more deeply. Maybe our cognitive abilities just
came from very comfy beds [2]?
Apes build comfy beds
But
there are other intelligent animals that definitely have no mattress to
sleep on. Many dolphins are able to rest one half of their brain while
the other one controls breathing, for which marine mammals have to come
to the surface. This phenomenon, which some of us would love to do
during boring seminars, is called uni-hemispheric slow-waves (USW). It
enables dolphins, whales and also many birds to rest one hemisphere at a
time with the contralateral eye closed, changing to the other one after
about two hours [1]. Brain waves similar to slow-wave sleep and REM
sleep in humans have been recorded in flying frigate birds which migrate
for several months at a time [3]. Remarkably, when in REM sleep, these
birds as well as some other mammals like the sperm whale also show
bi-hemispheric sleep. However, this behaviour can only be found on the
ground – or in the case of sperm whales while floating vertically in the
ocean, holding their breath for a long time [4]. A strategy like this
would never work in many sharks, since they need constant flow through
their gills to be able to breathe. Some seem to solve this problem by
doing “yo-yo diving“: First they swim to the surface to then glide
downwards, giving themselves a short period of rest [5].
Different Animals sleep differently
The
sleeping behaviour of terrestrial animals can also be very different
from ours: Does anyone sleep while standing - No?! Horses and other
bigger herbivores often do, with help of their stay apparatus, which
consists of ligaments and tendons that lock into place. Still, many of
them need to lie down for REM sleep as it comes with strong muscle
atonia. Due to this, horses, giraffes and also elephants actually end up
having much less sleep than we are used to, getting away with just 2-4
hours per day [6]. Many short naps seem to be more beneficial for these
animals, since it enables them to spend more time alert when predators are around.
Who
wouldn’t like to be capable of skipping 5-6 hours sleep per day in
order to prepare the next lab meeting? New-born orcas outperform us a
lot when it comes to little sleep: They stay awake for one full month
after birth, and only rest while pressing their body against their
mother. Compared to this, big brown bats and hairy armadillos lead a
pretty relaxed life, sleeping for roughly 20 hours per day [6].
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| Image Source: http://bit.ly/2B1GiM7 |
So
far we were mostly talking about sleep similar to human non-REM. But do
animals also dream? Of course, no one can ask a cat whether it was
recently dreaming of mice. Anyway, many mammals, birds and even reptiles
show physiological patterns with consistent with REM sleep. Dragonflies
have 350 REM cycles per day,
each of them lasting 80s [7] and the platypus spends approximately 5.8-8
h/day in REM [8]. REM sleep is sometimes seen as a key feature during
evolution of the amniote – the common ancestor of mammals, birds and
reptiles that lived more than 300 million years ago [7]. However,
octopus also seem to have REM like sleep patterns that go along with
changing colours and twitching of their arms [9].
Yet,
the animal kingdom consists of more than vertebrates. What about
insects, nematodes and porifera (sponges)? While for porifera there is
no evidence of sleeping behavior, the fruitfly D. melanogaster has
not only been shown to have sleep-like resting patterns but also
exhibit cognitive impairment upon sleep depriviation [1]. A fatigue
period (lethargus) before moulting in C. elegans suggests that sleep is somehow connected to development and related to neuronal changes [10].
What's the Purpose behind Sleep...?
Of
course most sleep studies focused on very few animals from each taxa so
far. For instance, just 50 out of 60000 vertebrate species have been
tested for all sleep criteria so far and not all of them were found to
meet all of the criteria. Nevertheless sleep-like behavior seems to be
present in various animals. Can we assume that all these animals sleep
for the same reason - and if so what is the reason?
Over
years, several theories on the function of sleep have been developed.
The original idea that sleep is mainly necessary to conserve energy
seems quite unlikely nowadays since it decreases metabolism by very
little amounts (5-10%), whereas hibernation saves a lot more energy
[11]. Another very prominent idea is that sleep is important for
learning and memory consolidation (see article of page 6). Even though
there is evidence for a role of sleep in memory, it is still disputed
what this role exactly is – ranging from memory deletion during REM
sleep to maturation of memory circuits [12]. Recently, sleep has been
linked with a restorative function in the central nervous system,
leading to a clearance of free radicals and other metabolic waste that
accumulates during wakefulness [13]. While many of these processes
definitely occur during sleep, they don’t explain the great variation in
sleep needs and patterns throughout the animal kingdom.
Do all animals sleep for the same reason?
Trying
to address this, some researchers now regard sleep as a state of
adaptive inactivity, optimizing the timing of behavior according to
prey/food availability and threats in the environment. In this scenario,
continuous wakefulness implies the greatest energy demands but
maximizes niche exploitation. This can explain why giraffes sleep so
little (they have a very low-caloric diet and a high threat of
predators). For bats that feed specifically on insects being active
between dusk and initial hours of darkness, on the other hand, a longer
period of time awake would be highly maladaptive since it increases
their risk of becoming prey [14].
... we don't fully know (yet)!
Exploring
sleeping behavior of other animals can, therefore, help to clarify its
function in humans. While sleep deprivation in humans and rodents so far
suggests that sleep influences cognition, emotion, immunity and memory,
the function of sleep can still be substantially different when looking
at all animals that exhibit sleep-like resting behavior. We should all
be aware that, as put by Michel Jouvet, a famous sleep researcher who just passed away, "it’s not enough to use the brain of your experimental animal, it’s also necessary to use your own”.
Annika Reinhold, MSc Student MedNeuro
References:
[1] Siegel, 2008, Trends in Neurosciences
[2] Shumaker et al., 2014, American Journal of Physical Anthropology
[3] Rattenborg et al., 2016, Nature Communications
[4] Miller et al., 2008, Current Biology
[7] Shein-Idelson et al., 2016, Science
[8] Siegel et al., 1999, Neuroscience
[10] Sing et al., 2013, Sleep
[11] Assefa et al., 2015, AIMS Neuroscience
[12] Diekelmann & Born, 2010, Nature Reviews Neuroscience
[13] Xie et al., 2013, Science
[14] Siegel, 2009, Nature Reviews Neuroscience
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