Welcome to a series the likes of which you've never seen before! Strong stories, highly charged with erotic content are all presented without Japan's export censorship - as they were intended to be seen.
Are there any animals that don't sleep?
tl;dr - Yes.
It really depends on how you define "sleep." Does sleeping include any daily (or otherwise periodic) period of inactivity? Or is there more involved?
I think it's fair to say that many animals don't sleep in the sense that we're used to.
When humans sleep, our inactivity gives our body to undertake tasks we can't do when awake.
Some scientists, like David Raizen, believe a main purpose of sleep is to allow for nervous system plasticity.
No matter what its purpose is, important neural changes occur during sleep.
Sleeping also helps reinforce immune function.
But many animals don't have well-developed nervous systems or complex immune systems. For example, sponges never sleep.
They don't even have real tissues, so why would they need to? Most cnidarians don't sleep either.
(Box jellyfish do enter into nightly periods of inactivity, but this is probably to limit energy loss during periods when they can't capture prey.
See "Do box jellyfish sleep at night?" by Seymour et al.) Raizen showed that nematodes like C.
elegans enter a stage at larval-stage transitions called lethargus, characterized by behavioral quiescence and other characteristics of sleep in other animals.
This period facilitates development and neural changes that are necessary in later life.
But this isn't a nightly, or even a periodic behavior; it's part of a life-cycle stage.
("Lethargus is a Caenorhabditis elegans sleep-like state" by Raizen et al.) Joan Hendricks has shown that D.
melanogaster also has certain periods (in some cases, daily) of behavioral quiescence that may have a common origin with that in nematodes.
If deprived of these periods, they become less responsive during periods of normal behavior.
I would guess that most insects will occasionally go into similar periods of dormancy.
Still, it's hard to say whether or not this really constitutes sleeping.
It certainly has most of its characteristics, but not enough research has been done into its purpose and physiological effects to say for sure how similar it is to human sleep.
(See here for more information: http://www.pennmedicine.org/slee...) Some insects and gastropods also estivate, but that's a separate process.
In fact, most animals have some kind of response system to lower metabolism in unfavorable conditions, but that hardly counts as sleeping, in my opinion. The earthworm Lumbricis terrestris is most active from dusk to dawn, and their oxygen consumption is lower during these less active periods, possibly indicating that they also have less neural activity.
Presumably, some other annelids share this feature of being most active at certain periods.
But again, it's a matter of judgment whether you want to consider this sleeping.
See Kevin Butt's work for more info. That's just a sampling of the many animal phyla.
Irene Tobler wrote in Scientific American that sleep may have evolved to allow more flexibility from rigid circadian rest-activity rhythms.
Most vertebrates sleep, she says, also our knowledge of the fish and amphibia is limited.
In many invertebrates, a rest state similar to sleep has been identified.
However, more research needs to be done, she says, in some taxa like molluscs.
Key section excerpted below, illustrating the difficulties of defining "sleep": In addition to a circadian component, sleep in vertebrates is characterized by a homeostatic component: sleep is regulated in its intensity as a function of the duration of previous wakefulness.
Experiments have elicited a homeostatic response to some hours of "rest deprivation" in scorpions and several cockroach species--that is, they were more restful during recovery.
[...] It seems that the elementary features which characterize sleep in its most evolved state--as it is found in mammals and birds--are already present even in very primitive organisms.
At all phylogenetic levels, scientists are faced with the challenge of identifying exactly what aspects of the organism are "restored" during sleep.
Once these are found, we may be able to provide a meaningful answer to the question of whether unicellular organisms (bacteria, for instance) sleep. Read the full article here: http://www.scientificamerican.co...