Sleep is something (most of us) spend a third of our lives doing, though we hardly give it a second thought. Yet sleep is absolutely essential for all vertebrates, and if an animal is completely deprived of sleep, it will die. Rats that are sleep deprived die in a matter of days, and in the past, when it was still possible to do crazy human experiments, humans have been kept awake for as long as 72 hours, within which time they would descend into psychosis. The few human sleepless anomalies who exist remain with numerous mental and metabolic problems, and the reason why they remain alive are unanswered (warning: do not try to avoid sleeping completely for days on end in order to discover that you are a natural anomaly. You WILL die).
One of the most fascinating questions that still remains somewhat unanswered is “why did sleep evolve in animals”? Now, single celled organisms (like bacteria or yeast) don’t sleep. They continue to grow, and constantly divide at a certain rate. More interestingly, our own cells, when isolated and grown independently in culture, don’t “sleep”, but continue to divide and grow constantly. Sleep itself is a complex phenomenon with distinct physiological, neurological and psychological features. Evolutionarily, one could reason that it could even be advantageous for an animal to evolve to avoid sleeping completely. This would give that animal twice the amount of time needed to forage for food, or reproduce (compared to other competitors), and could even be considered a huge survival advantage. But that has not happened, suggesting that the evolutionary need for sleep is far greater than any benefit a lack of sleep could allow.
Sleep remains essential for all animals, and there remain a number of reasons why that could be so. One reason is that as organisms became more complex (and evolved into multicellular organisms from single celled ones), the need to adapt to a day-night cycle on earth became stronger. This is what’s known as the circadian clock. Much progress has been made in understanding the molecular mechanisms of the circadian clocks, and identifying master-regulators of this clock (like the clock, period and BMAL genes). The subsequent consequences on the cell cycle and metabolism are slowly being unraveled. Yet, the circadian clock does not answer the need for sleep (though understanding the clock allows us to understand many aspects of basic metabolism and growth), since some single cellular organisms that don’t “sleep”, like cyanobacteria, exhibit a robust circadian clock.
Other researchers look at sleep and the circadian clock itself from a metabolic perspective, and from the basic metabolic needs of the body. During sleep, some dramatic processes occur, starting with the basic metabolism of the animal, which shifts from catabolism (or breakdown of molecules, and the release of energy) to anabolism (the active consumption of energy, and subsequent growth and building). So, sleep is in some ways the opposite of a “resting state”, as energy is being consumed, in order to let the body grow or build or recover.
All these details are slowly being unraveled. Yet understanding the very fundamental question of “why we sleep, and how sleep evolved” remains one of the great unanswered questions of science. In a recent post, I described Dan Koshland’s concept of discovery; Charge, challenge and chance. Understanding the fundamental need for sleep, and the evolutionary reasons for it will remain one of the great science questions which will perhaps be answered by “charge”, or “challenge”.