On the Function and Evolution of Sleep

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On the Function and Evolution of Sleep

If physiologists devoted the most research time to behaviors humans engaged in the most, we would probably have a full understanding of the biological purpose of sleep. After all, humans, with the exception of most college students, spend one third of their lives in a somnolent state. Despite its fundamental role in human and animal life, sleep is, even in an age when neuroscience has reduced many behaviors to neurological mechanisms, still quite mysterious. What processes are taking place during sleep that benefit the organism? Why spend so much time in an unresponsive and vulnerable state? That these questions haven't been definitively answered is really not a function of a lack of effort on
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In 1995 researchers found the first evidence of neurons involved in both temperature regulation and sleep (1). A team discovered neurons in the front part of the hypothalamus of cats which, when warmed by two degrees Celsius, fire more rapidly. These neurons also increase their firing frequency when the cats sleep (1). The researchers attributed a dual role to the neurons in this region. These dual functioning neurons may be living evidence of an ancient transition from mere temperature maintenance to actual sleep.

The origin of sleep seems to be closely related to the evolution of mechanisms of enhancement and maintenance of synaptic capability. Roffwarg, Musio, and Dement conjectured over thirty years ago that the function of spontaneous, repetitive excitations of neural circuits during REM sleep in human embryos is to facilitate circuit development and maintenance (2). This concept provided the foundation for the "dynamic stabilization" (DS) paradigm of neural circuitry. According to this model, frequent synaptic activation enhances synaptic strength in neural circuits storing inherited information, or "phylogenic memories," and information acquired through experience, "ontogenetic memories." (2) DS can occur either through regular functional use or by way of spontaneous oscillatory neural activity. The spontaneous activations do not actually