Sleep and Long-Term Memory: Could That Be the Reason We Sleep?

We know we require sleep for our survival, but we've yet to discover the why of sleep. A ground-breaking new study might help us answer this and many more questions.
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Here's a fact that might surprise you: we really do not know why we sleep. Of course, we sleep because our bodies demand it; we know we require sleep for our survival. But we've yet to discover the why of sleep, to determine the physiological purpose that sleep serves. Sleep is one of our most elemental functions, essential to life, and the purpose of sleep remains something of a mystery.

That's why this news is so exciting, and potentially important: a new study has found a direct link between sleep and the creation of long-term memories.

Scientists at Washington University School of Medicine have discovered a cluster of cells in the fruit-fly brain that controls sleep. By manipulating those cells, scientists were able to establish long-term memories in fruit flies by controlling their sleep habits. How did they accomplish this? First, the researchers bred genetically modified fruit flies to sleep on demand. Using their ability to control the fruit flies' sleep, the scientists tested the insects' ability to learn -- and retain -- information. Here's how they did it, and what they found:

  • As a test, researchers exposed male fruit flies to other male flies that had been engineered to smell like female fruit flies.

  • After a few unsuccessful mating attempts, the flies learned not to court these female-in-disguise flies.
  • Without sleep, the fruit flies retained this knowledge of the pretend-female flies for a short period of time, amounting to a few hours.
  • Scientists put their fruit-fly subjects to sleep after the courtship training. With sleep, the fruit flies were able to retain the same information for several days. Sleep enabled the fruit flies to convert short-term knowledge into long-term memory.
  • We've long known that there was a relationship between sleep, memory and learning. You don't have to be a scientist to have a sense of this. Think about your typical state of mind -- and your inclination to retain new information -- at the end of a long, busy day.

    The science behind your end-of-day brain fatigue is also what the results of this new research appear to confirm: a theory called synaptic homeostasis. Like all animals, fruit flies included, our brains are engaged in processing information every moment we're awake. The synapses in our brains are a key component of this process. Synapses create communication pathways in the brain that enable us to retain information. The theory of synaptic homeostasis suggests that sleep functions like a filter, to help us weed out and relax the synapses we develop over the course of a day, in order to start fresh the next day. Our brains use sleep as the time to determine what information can be discarded, and what is useful enough that it should be stored as longer-term memories.

    In their fruit-fly subjects, researchers discovered:

    • Flies in stimulating, learning-rich environments created more synapses than flies kept in isolation.
  • During sleep, these synapses were reduced in size and number, essentially clearing out the clutter in the brain to prepare for another round of learning.
  • So, what are the implications for us humans? A sleep-inducing switch for our brains sounds like the stuff of science fiction, and its safe to say we're a ways away from this. But this is a dramatic step toward developing an answer to that elusive question of why we sleep. The more we understand about the underlying reasons for sleep, the better able we'll be to explore and develop safe, natural solutions to sleep problems.

    Sweet dreams,

    Michael J. Breus, Ph.D.
    The Sleep Doctor™
    "Everything you do, you do better with a good night's sleep"™

    Find Dr. Breus on Twitter @thesleepdoctor, and on Facebook at

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