Sleep as the Default State

Just read an interesting essay* which changed my thinking about the role of sleep.  While nobody can claim to understand exactly why sleep is necessary for mammals, most of the explanations focus on some positive, regenerative benefits that we can’t do without (e.g. maintaining the neuronal circuitry).  Martin Kinsbourne puts forth another benefit, which I’d never thought of:

Sleep conserves energy and keeps animals out of trouble.  It takes members of each species a minimum time per day to make a living — that is, to secure their personal survival and take advantage of any reproductive opportunity.  This challenge is met anew every day.  On this view, how much of the day is needed to meet adaptive goals determines the duration of the default option of sleep.

The idea that sleep is the default state, and that being awake requires a lot of hard work and danger is one that never occurred to me.  Now, it’s also true that it can be dangerous for tasty, defenseless animals to be asleep, but moving around looking for food and mates certainly attracts more attention.  Assuming I’m able to get enough food in one hour a day and find a rock to hide under, maybe it makes evolutionary sense for me to sleep the other 23 hours.

Another essay in the same book (Amazon Search Inside on “Strogatz”) points to research which roughly correlates the number of hours an animal sleeps a day with brain size.  The supposition is that whatever happens to change the brain during the day requires corresponding down time to maintain proper function.

But what if the real reason for the correlation is that animals with bigger brains tend to be able to defend themselves better against predators because they are smarter?  Part of a good predatory defense is just not being caught in a position where you are likely to get eaten, which requires the ability to predict the future to some degree.  Plus, if you are caught in a chase where you are slower than your opponent, you’ll need to use your clever brain to find or create shelter during the chase.

It would be foolish to assume that the brain size / sleep correlation is due to just one factor, but this “default state” hypothesis suggests an interesting experiment: see if sleep duration correlates with how defenseless an animal is.  You’d have to be careful to define defensive abilty a priori so as not to fall into a circular reasoning trap.  Just off the cuff, I’d suggest it is at least a cross product of speed, innate defenses (such as shell, sharp spines, poisonous secretions, etc.), camouflage, scary markers (e.g. mimicking a predator), and sociality (the ability to use intraspecies teamwork to warn, fight, confuse, or survive through probabilities).

Anyone know of research along these lines?



* To read the essay online, go here and type “Kinsbourne” into the search field.