Why do we sleep?

Although several hypotheses attempt to explain why we sleep, it remains a scientific mystery. Photo courtesy of Photocase.com.

Most of us spend about 7-9 hours of the day in a passive state of unconsciousness. Why do we humans have to ‘waste’ around a third of our lives doing absolutely nothing? This time could surely be better used for something else. However, like everything else in the world that surrounds us, sleep has been subjected to evolutionary forces, so since it is observed universally among animals, its advantages must outweigh the costs of less time available for food and mate searching. We know from ourselves that we definitely need sleep, without it, we become drowsy and our mental and motor faculties lose efficiency. With prolonged sleep deprivation we start to hallucinate and lose grasp of reality. The same effects are seen in animals. However, science cannot yet completely solved the mystery of we have to sleep. In a recent review paper, Emmanuel Mignot from the Howard Hughes Medical Institute looks closer at the hypotheses for why we sleep. 

Sleep periods can be separated into deep dreaming REM (rapid eye movement) sleep and lighter non-REM sleep. It is a lot easier to wake up animals and humans when they are not in the REM sleep period. Migrating and swimming animals, as well as animals experiencing a high predation pressure show only short period of REM-sleep and some animals are even able to sleep with only one brain half at a time.

Mignot describes three major hypotheses of the advantages of sleep.

The sleep as energy conservation hypothesis says that sleeping evolved as a way of saving energy at time (usually at nights) when it was more difficult to find food. This hypothesis gets some support from studies of ecological niches, but it does not explain the occurrence of REM sleep, which is much more energy demanding that light sleep. The increased vulnerability during sleep may also very well offset the advantages gained by saving energy.  

The sleep as facilitating memory and synapse plasticity hypothesis says that sleeping evolved as a way of cleaning up in the learning episodes and synaptic changes made during the day, so that only the important events are stored in memory. This claim is supported by the obvious signs of mental disturbances seen in sleep deprived humans and mammals. However, it does not explain the need for sleep in more primitive animals or that memory and learning can occur without sleep.

The sleep as macromolecule restoration hypothesis says that the cellular components needed for making macromolecules (used in cholesterol and protein synthesis and in intracellular transport for instance) are depleted during the day and needs to be replenished as night when the need for macromolecules are nonexistent. This hypothesis satisfactorily explains why sleep in necessary in all animals, but again does not explain the presence of REM sleep.

None of these hypotheses has yet gained widespread support and indeed it may be that none of them alone can answer the question of why we sleep. Thus sleeping remains a scientific mystery at least until more data is procured.

Source
Mignot E (2008) Why we sleep: The temporal organization of recovery. PLoS Biol 6(4): e106. doi:10.1371/journal.pbio.0060106

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