5 JANUARY 2006
have long dreamed of discovering the reason why animals need to sleep.
It's an old mystery. After all, from a survival standpoint, sleeping seems pretty stupid. You could acquire more resources, and be less exposed to danger, by remaining active and alert 24/7.
Similar reasoning applies to cities. Urban businesses could generate more economic activity by operating 24/7 also. Yet cities (except for New York, New York) sleep as well.
"Just as with complex organisms, a city has a periodic cycle of activity, characterized by
high levels of activity during the day, followed by periods of relatively low levels of activity at night," observes Emmanuel Tannenbaum, a bioscientist at Ben-Gurion University of the Negev in Israel.
And just why does a city sleep? Because people do, Tannenbaum observes. People are the individual agents constituting a city, just as neurons (nerve cells) are the agents constituting a brain. So perhaps brains need to sleep because their neurons need to sleep, too.
This suggestion stems from an old idea proposed by Francis Crick, co-discoverer of DNA's double-helix structure. Crick supposed that brains needed a time to dispose of the "garbage" produced during normal activity.
"Sleep is a time when the brain sorts through various stored memories, and discards those deemed unessential, while processing those deemed essential for long-term storage," Tannenbaum explains in a new paper.
And in fact, he notes, recent evidence supports Crick's idea. Studies show that neurons accumulate a protein called Fos during periods of stimulation. Apparently Fos builds up as neurons go about performing their normal tasks. At some point, Fos levels become so high that normal activity is hindered, kind of like sooner or later a messy office makes it hard to find your computer keyboard. During sleep, though, Fos levels recede, cleansing the neuron in preparation for the next day's work.
Aquatint by Francisco de Goya: "El Sueño de la razon produce monstruos (The sleep of reason brings forth monsters)" c. 1797. From Artcyclopedia
At first, you might think that neurons could simply shut down and dispose of their excess Fos independently, so the rest of the brain could go on working around the clock. But that would be like businesses in a city shutting down at random to clean up their desks. You can't do business efficiently if suppliers of products and services aren't reliably available. So it makes sense for everybody (except Denny's) to shut down at the same time -- during the night.
In a similar way, the brain is a highly interconnected complex system, any one part depending on signals and information from many others to conduct its business. If neurons shut down for cleanup any time they wanted, some parts of the brain would be impaired. (Perhaps you've noticed that when you're sleepy, you don't think straight.) The efficient solution is for all the neurons to shut down at the same time. So the whole brain has to go to sleep.
"If some neurons were to engage in garbage collection independently of others, this would impede the proper functioning of other neurons, who may rely on the 'sleeping' neurons for important information," Tannenbaum points out.
This is not merely a clever idea for explaining sleep. It might reflect an even deeper insight into the nature of life in general. Cities evolved as agents (people) interacted and self-organized into efficient economic units that became agents interacting in a larger economy; a complex brain evolved from the self-organization of agents (neurons) into an efficient thinking machine, guiding the interaction of new agents (people). In both cases, agents at one scale of size interacted in a collective way to produce a new "agent" at a larger scale of size. Complex life itself, Tannenbaum believes, may have originated as a consequence of the same principle.
That idea, described in an earlier paper by Tannenbaum, meshes nicely with the popular view that today's DNA-based life descended from DNA's molecular cousin, RNA. Versatile versions of RNA molecules, many experts believe, catalyzed their own reproduction in an "RNA world" that only later evolved into a system of DNA genes guiding the construction of proteins, with RNA serving as a mere messenger.
In recent years, networks of small RNA molecules have been discovered within the cells of complex organisms, suggesting (to Tannenbaum, at least) that today's complex cells may be the large-scale agents built from small-scale RNA agents. In other words, a human cell houses an "RNA community." And so far from being a relic of the distant evolutionary past, the RNA world may still be thriving within DNA-driven cells -- cells that are basically vehicles that RNA created to contain its community.
If so, the networks of small RNA molecules in today's cells may be the living fossils of the original RNA world. Our supposedly advanced "eukaryotic" cells could be ancient time machines preserving primitive life's RNA origins. At the very least these small RNAs may hold clues to the planet's first living molecular species.
Speculative as these proposals are at the moment, they represent the kind of reasoning that offers hope of progress in solving mysteries that have long remained resistant to science's probing. Solving old mysteries usually requires new ways of thinking -- provided by wide-awake brains open to far-out ideas.
E-mail: tsiegfried@nasw.org
