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  &#8220;Lack of sleep disrupts every physiologic function in the body,&#8221; said Eve Van Cauter of the University of Chicago. &#8220;We have nothing in our biology that allows us to adapt to this behavior.&#8221;
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  The amount of necessary sleep varies from person to person, with some breezing through their days on just a few hours&#8217; slumber and others barely functioning without a full 10 hours, experts say. But most people apparently need between about seven and nine hours, with studies indicating that an increased risk for disease starts to kick in when people get less than six or seven, experts say.
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  So why do mammals and birds have REM sleep at all? &#8220;The best answer I can come up with is that it&#8217;s there to prepare you for waking,&#8221; Dr. Siegel said. &#8220;When the important work of sleep is done, REM sleep just makes you as alert as you can be while you&#8217;re asleep.&#8221;
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  They found that meditating actually increases the thickness of the cortex in areas involved in attention and sensory processing, such as the prefrontal cortex and the right anterior insula.
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  “You are exercising it while you meditate, and it gets bigger,” she says. The finding is in line with studies showing that accomplished musicians, athletes and linguists all have thickening in relevant areas of the cortex. It is further evidence, says Lazar, that yogis “aren’t just sitting there doing nothing&#8221;.
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  The growth of the cortex is not due to the growth of new neurons, she points out, but results from wider blood vessels, more supporting structures such as glia and astrocytes, and increased branching and connections.
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