The Universe of Discourse

Wed, 18 Jan 2006

Daniel Dennett on sleep

Daniel Dennett is a philosopher of mind and consciousness. The first work of his that came to my attention was his essay "Why You Can't Build a Computer That Can Feel Pain". This is just the sort of topic that college sophomores love to argue about at midnight in the dorm lounge, the kind of argument that drives me away, screaming "Shut up! Shut up! Shut up!"

But to my lasting surprise, this essay really had something to say. Dennett marshaled an impressive amount of factual evidence for his point of view, and found arguments that I wouldn't have thought of. At the end, I felt as though I really knew something about this topic, whereas before I read the essay, I wouldn't have imagined that there was anything to know about it. Since then, I've tried hard to read everything I can find that Dennett has written.

I highly recommend Dennett's 1995 book Darwin's Dangerous Idea. It's a long book, and it's not the main point of my essay today. I want to give you some sense of what it's about, without straining myself to write a complete review. Like all really good books, it has several intertwined themes, and my quoting can only expose part of one of them:

A teleological explanation is one the explains the existence or occurrence of something by citing a goal or purpose that is served by the thing. Artifacts are the most obvious cases; the goal or purpose of an artifact is the function it was designed to serve by its creator. There is no controversy about the telos of a hammer: it is for hammering in and pulling out nails. The telos of more complicated artifacts, such as camcorders or tow trucks or CT scanners, is if anything more obvious. But even in simple cases, a problem can be seen to loom in the background:

"Why are you sawing that board?"
"To make a door."
"And what is the door for?"
"To secure my house."
"And why do you want a secure house?"
"So I can sleep nights."
"And why do you want to sleep nights?"
"Go run along and stop asking such silly questions."

This exchange reveals one of the troubles with teleology: where does it all stop? What final cause can be cited to bring this hierarchy of reasons to a close? Aristotle had an answer: God, the Prime Mover, the for-which to end all for-whiches. The idea, which is taken up by the Christian, Jewish, and Islamic traditions, is that all our purposes are ultimately God's purposes. . . . But what are God's purposes? That is something of a mystery.

. . . One of Darwin's fundamental contributions is showing us a new way to make sense of "why" questions. Like it or not, Darwin's idea offers one way—a clear, cogent, surprisingly versatile way—of dissolving these old conundrums. It takes some getting used to, and is often misapplied, even by its staunchest friends. Gradually exposing and clarifying this way of thinking is a central project of the present book. Darwinian thinking must be carefully distinguished from some oversimplified and all-too-popular impostors, and this will take us into some technicalities, but it is worth it. The prize is, for the first time, a stable system of explanation that does not go round and round in circles or spiral off in an infinite regress of mysteries. Some people would very much prefer the infinite regress of mysteries, apparently, but in this day and age the cost is prohibitive: you have to get yourself deceived. You can either deceive yourself or let others do the dirty work, but there is no intellectually defensible way of rebuilding the mighty barriers to comprehension that Darwin smashed.

(Darwin's Dangerous Idea, pp. 24–25.)

Anyway, there's one place in this otherwise excellent book where Dennett really blew it. First he quotes from a 1988 Boston Globe article by Chet Raymo, "Mysterious Sleep":

University of Chicago sleep researcher Allan Rechtshaffen asks "how could natural selection with its irrevocable logic have 'permitted' the animal kingdom to pay the price of sleep for no good reason? Sleep is so apparently maladaptive that it is hard to understand why some other condition did not evolve to satisfy whatever need it is that sleep satisfies.

And then Dennett argues:

But why does sleep need a "clear biological function" at all? It is being awake that needs an explanation, and presumably its explanation is obvious. Animals—unlike plants—need to be awake at least part of the time in order to search for food and procreate, as Raymo notes. But once you've headed down this path of leading an active existence, the cost-benefit analysis of the options that arise is far from obvious. Being awake is relatively costly, compared with lying dormant. So presumably Mother Nature economizes where she can. . . . But surely we animals are at greater risk from predators while we sleep? Not necessarily. Leaving the den is risky, too, and if we're going to minimize that risky phase, we might as well keep the metabolism idling while we bide our time, conserving energy for the main business of replicating.

(Darwin's Dangerous Idea, pp. 339–340, or see index under "Sleep, function of".)

This is a terrible argument, because Dennett has apparently missed the really interesting question here. The question isn't why we sleep; it's why we need to sleep. Let's consider another important function, eating. There's no question about why we eat. We eat because we need to eat, and there's no question about why we need to eat either. Sure, eating might be maladaptive: you have to leave the den and expose yourself to danger. It would be very convenient not to have to eat. But just as clearly, not eating won't work, because you need to eat. You have to get energy from somewhere; you simply cannot run your physiology without eating something once in a while. Fine.

But suppose you are in your den, and you are hungry, and need to go out to find food. But there is a predator sniffing around the door, waiting for you. You have a choice: you can stay in and go hungry, using up the reserves that were stored either in your body or in your den. When you run out of food, you can still go without, even though the consequences to your health of this choice may be terrible. In the final extremity, you have the option of starving to death, and that might, under certain circumstances, be a better strategy than going out to be immediately mauled by the predator.

With sleep, you have no such options. If you're treed by a panther, and you need to stay awake to balance on your branch, you have no options. You cannot use up your stored reserves of sleep. You do not have the option to go without sleep in the hope that the panther will get bored and depart. You cannot postpone sleep and suffer the physical consequences. You cannot choose to die from lack of sleep rather than give up and fall out of the tree. Sooner or later you will sleep, whether you choose to or not, and when you sleep you will fall out of the tree and die.

People can and do go on hunger strikes, refuse to eat, and starve to death. Nobody goes on sleep strikes. They can't. Why not? Because they can't. But why can't they? I don't think anyone knows.

The question isn't about the maladaptivity of sleeping itself; it's about the maladaptivity of being unable to prevent or even to delay sleep. Sleep is not merely a strategy to keep us conveniently out of trouble. If that were all it was, we would need to sleep only when it was safe, and we would be able to forgo it when we were in trouble. Sleep, even more than food, must serve some vital physiological role. The role must be so essential that it is impossible to run a mammalian physiology without it, even for as long as three days. Otherwise, there would be adaptive value in being able to postpone sleep for three days, rather than to fall asleep involuntarily and be at the mercy of one's enemies.

Given that, it is indeed a puzzle that we have not been able to identify the vital physiological role of sleep, and Rechtshaffen's puzzlement above makes sense.

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