By Deepak Chopra, M.D., FACP, and P. Murali Doraiswamy, MBBS, FRCP
In the last post we saw how essential sleep is for the physiological needs of almost all animals, but sleep would seem to be a poor survival trait as far as evolution goes. Because sleep put our ancestors (and other living creatures) at risk from predators, the benefits must outweigh the risks. That's all that scientists can manage to agree on. Unlike humans, some animals (e.g., newborn dolphins) can survive sleep deprivation for a couple of weeks without apparent harm. However, in most species, after extended sleep deprivation their body temperature and metabolism become unstable, and they die. The longest period a human has survived sleep deprivation is believed to be about two weeks, but many physical and mental deficits occur long before that; driving ability is significantly impaired after one night's bad sleep.
Since the brain remains active in sleep, some of the proposed roles for sleep make it into a time to restore, repair, conserve energy, or all of the above. Besides the mouse study cited previously, the fact that toxins build up during sleep deprivation also suggests that the human brain clears toxicity in sleep. Another clue: Sleep increases the number of repair cells (oligodendrocytes) in the brain. Even though Freud's dream theories have been discredited, which has also led to total disagreement about the psychological import of dreams, sleep may be a time for rehearsing what we learned during the day and archiving our memories -- much like the librarian who works after the library is closed to reshelve books so that they are easy to find the next day.
As things stand, we know considerably more about sleep deprivation than about sleep itself. Subjects deprived of sleep do poorly on a variety of cognitive tests, and sleep deprivation is clearly linked to more accidents and errors. People underestimate how much is lost by having bad sleep. There is evidence that no daily rhythm is more important. It used to be thought that sleeping in on the weekends doesn't actually work to overcome sleep deficit during the week, but this has been challenged recently by a finding that a sleep-deprived person can enter REM sleep with a short nap, not the hours that researchers once thought were necessary. The best guess now is that you can be fully alert for five or six hours right after you wake up from less than eight hours' sleep, but that after this grace period, deficits will begin to show up.
Finally, sleep is related to mood -- strangely enough, sleep deprivation can make people happy and sometimes manic. Decades ago doctors took advantage of this fact in trying to treat depression (a misguided strategy, now that we recognize the link between depression and bad sleep). For some reason, people with depression often go into REM sleep (and dreaming) more quickly than normal. But if that is symptomatic, a different link suggests that dreams can lead to creative insights, not to mention the kind of dreams that make us feel elated.
Numerous creative breakthroughs have been attributed to dreams, such as the tune for the Beatles song "Yesterday" (Paul McCartney), the structure of carbon and benzene (August Kekulé), and the sewing machine (Elias Howe). Indeed, the discovery of acetylcholine, a chemical that regulate many aspects of dream sleep, reportedly came to Otto Loewi in dreams on two consecutive nights in 1921. On the first night he woke up and scribbled down some notes in his diary that, alas, he couldn't read in the morning. On the second night he was lucky enough to write them more legibly. Loewi's subsequent experiment based on his dreams won him a Nobel Prize.
Dreams can be induced by taking certain drugs (e.g., ones that boost acetylcholine or serotonin) or withdrawing other drugs (e.g., sleeping pills), so we know that dreams have a chemical mechanism. Likewise, we know that many commonly used substances, such as alcohol, cough-cold syrups, some blood pressure medicines, antidepressants, and energy drinks, suppress REM sleep, although no one has looked at how this affects long-term creativity or memory. In a rare condition called familial fatal insomnia people lose their ability to sleep and develop a form of early-onset dementia.
Do we have to restrict dreams to one purpose only, whatever current theories think it may be? In various cultures and seen through different eyes, dreams are prophetic, messages from the gods, the tickling of the muse, random neuronal firings, and of course Freud's "royal road to the unconscious." And we have no way to determine categorically if other species also dream, although they must, given that REM sleep occurs in so many mammals. (Who hasn't seen a dog growl and writhe in its sleep and surmised that it must be chasing a cat in its dreams?)
So despite five decades of modern neuroscience, we have only a very limited knowledge of the role of sleep and barely know anything about the role of dreams. Common experience tells us to agree with Shakespeare's simple conclusion that sleep "knits up the raveled sleeve of care." Readers of ancient Vedic texts from India may accept that sleep is a time for inward knowing and subtle experiences in consciousness. Modern skeptics may declare that no such esoteric knowledge exists and that dreams are junk bins for the detritus of the working brain. Without a fuller understanding of consciousness itself, however, such arguments are adrift in the same darkness we inhabit every time we fall asleep.
Deepak Chopra, M.D., is the author of more than 75 books, with 22 New York Times bestsellers.
P. Murali Doraiswamy, MBBS, FRCP, is Professor of Psychiatry at Duke University Medical Center in Durham, N.C., and a leading physician scientist in the areas of mental health, cognitive neuroscience and mind-body medicine.