Sleep remains one of biology's most confounding mysteries. We are just beginning to understand how it evolved, but defining this strange state of consciousness is incredibly difficult — especially because there are so many ways to experience it.
Humans obviously love sleep. Like all mammals, we literally can't live without it. But when did our capacity for, and dependence upon, sleep evolve? On a related note: how common a phenomena is it, really? Can we say, for example, whether organisms like bacteria sleep in the same way that humans do?
You can carry on with this line of inquiry til your eyelids get heavy. Do fish require slumber? Do invertebrates ever nod-off inadvertently? Regarding bacteria: are microorganisms even complex enough to require shut-eye, seeing as they have no eyes to shut?
The answer to these questions and others like them ultimately boils down to how we study and define sleep. One way to do this is through behavior. A sleeping organism will often adopt a characteristic posture (curling into a ball, for example) or experience changes in breathing, and is typically unresponsive to low-grade external stimuli; the droning voice of an aggressively dreary college lecturer, or the gentle prod of a fellow classmate, is probably as useless at rousing a snoozing human as it is any other "sleeping" animal. A more technical definition of sleep might rely on physiological measurements of brainwave patterns or eye movements. These definitions help researchers distinguish between states of activity, rest, and sleep – though the nature of these states tend to vary considerably between organisms.
Take vertebrates, for example. As biologist and long-time sleep researcher Irene Tobler-Borbély notes, the mechanisms characteristic of avian and mammalian slumber are very much alike, in that they experience similar changes in brain waves typically associated with periods of unconscious rest. Birds, like mammals, experience both rapid and non-rapid eye movement (REM and NREM, respectively, which together constitute several stages of sleep), and the EEG-measured neuronal activity characteristic of each.
Sleep in reptiles, however, looks a little different (at least at the level of brain activity), and sleep in fish and amphibians, while under-studied, is probably even more dissimilar; some species of fish, for example, may not sleep at all – which throws a wrench in the works of the widely assumed notion that "every creature on the planet sleeps."
That being said, what these and other species (invertebrates included) do share in common with mammals and birds are behaviors characteristic of sleep-like states. They become motionless. Their heart-rate slows. Perhaps most telling of all: if an animal is deprived of sleep, or kept from entering its sleep-like state, it tends to compensate for the loss by resting more intensely when finally given the opportunity – the same way you're liable to crash extra hard after pulling an all-nighter.
The ubiquity of sleep and sleep-like behavior across numerous branches of the tree of life suggests that sleep, in the words of psychologist Ray Meddis, "is of great evolutionary age." This supports the hypothesis that some ancient organism – a common ancestor to a wide range of life on Earth today – probably experienced some primordial version of putting its feet up and passing out in a slack-jawed stupor. If you count the light-driven biochemical cycles common among organisms with circadian rhythms (i.e. pretty much every organism), even plants could be said to "sleep," indicating that this common ancestor could predate the division of some of life's most basic categories.
Evidence of sleep's deep evolutionary roots have been further supported by the advent of genetic sequencing. From a 2008 study by neuroscientists David Raizen and John Zimmerman, along with their colleagues at UPenn's Center for Sleep & Respiratory Neurobiology:
The past ten years have seen new approaches to elucidating genetic pathways regulating sleep. The emerging theme is that sleep-like states are conserved in evolution, with similar signaling pathways playing a role in animals as distantly related as flies and humans.
If some late, great evolutionary ancestor slept/rested/relaxed/what-have-you, it seems logical to ask whether unicellular organisms like bacteria sleep. The short answer is: we simply don't know – at least not by our current understanding and definitions of sleep. To date, the most primitive organism in which sleep-like states have been said to occur is Caenorhabditis elegans, a nematode roundworm (pictured below) with a nervous system comprising just 300 neurons.
In 2008, Zimmerman and Raizen published a study in Nature, wherein they observed in C. elegans a state of rest that they dubbed "lethargus." Similar to the sleep you and I experience, C. elegans in lethargus exhibited reduced responsiveness, but could be roused from their "slumber." The researchers did the rousing by repeatedly poking the C. elegans in the butt with an eyelash (seriously), in some cases for well over an hour. The nematodes that were deprived the opportunity to slip into lethargus (again with the butt-poking) were also shown to settle down for longer periods after being robbed of rest.
Tellingly, lethargus usually precedes molting, an energetically costly process by which the roundworm sheds its outer layer. "The association of this C. elegans sleep-like state with developmental changes," write the researchers, "suggests that sleep may have evolved to allow for developmental changes."
Whether unicellular organisms like bacteria sleep, therefore, may ultimately depend on our understanding of why sleep occurs in the first place. Theories on this subject abound. A leading hypothesis posits that sleep is somehow tied to our ability to establish and recall memories. Other researchers maintain, somewhat controversially, that we sleep not to remember, but to forget. Neuroscientists of a more diplomatic ilk might contend that organisms sleep simply to "restore" themselves, though this is usually at the expense of specifics; asking what it is that gets restored, exactly, is usually a good way to resume stoking the coals of debate.
These disagreements give rise to a sort of chicken and egg scenario that pits definitions of sleep against why we sleep in the first place, and it's a rather confounding scenario, at that. Sleep – after literally centuries of research – remains one of the most poorly understood areas of biology. In the words of William Dement, a pioneer in the field of sleep research and founder of Stanford University's Sleep Research Center:
“As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.”