- Sleep plays a crucial role in flagging and storing important memories (New York Times, Oct 2007 via Psych Central)
- Sleep appears to have a dramatic impact on the way the brain functions the next day (BBC News, July 2008)
Alternatively, we can understand the purpose of sleep by observing the effects of sleep deprivation.
- In drivers and workers it can lead to fatal accidents. In those under interrogation it can lead to confession. (Economist July 2006)
- Sleep Loss Challenges Emotional Control (Psych Central, Oct 2007)
- No sleep 'renders brain erratic' (BBC News, May 2008)
And yet hundreds of articles on sleep begin with some statement about the difficulty of understanding its purpose.
- Philosophers, neuroscientists and researchers have struggled with this seemingly simple question for centuries. (Psych Central)
- The real nature and purpose of sleep has long puzzled scientists. (Science Clarified)
- The purpose of sleep is not entirely understood. (Dr Spock)
So where are the difficulties? For a start, there are some patterns of human sleep that don't fit the simple explanations.
- Less sleep may cure tiredness (BBC News, July 2002)
- Life's too short to sleep? Blame the 'Thatcher' gene (TimesOnline, April 2005)
- Gene determines sleep demands (BBC News, March 2007)
Meanwhile sleep (or something akin to sleep) can be found in a wide range of organisms that don't have memories or emotions quite like ours.
- Round Worms Help Explain Purpose Of Sleep (RedOrbit, January 2008)
Biologists would have us believe that the purpose of sleep is coded into some genes, shared between humans and other species. In evolutionary terms, the phenomenon of sleep appears to predate the kinds of effect we started with. Maybe humans are a bit ratty and accident prone after a disturbed night, but worms aren't. So we shouldn't infer the purpose of sleep from human effects alone; if there is a good explanation for sleep, it isn't necessarily a human-centric one. POSIWID may still give us important clues, but we need to interpret them carefully.
At least POSIWID gives scientists reasonable hypotheses to explore. Sometimes the only apparent alternative is a heuristic explanation, but Bateson and his followers (Watzlawick, Keeney) thought such explanations were like the empty explanation mocked by Moliere - explaining sleep by the presence of something called The Dormitive Principle. Bobby Matherne attributes a similar critique of empty explanation to Rudolf Steiner.