Sleep occupies roughly one-third of our lives, yet the scientific community is still not entirely clear on its purpose or function. Existing data point most strongly to its role in memory and homeostasis: that sleep helps maintain basic brain functioning via a homeostatic mechanism that loosens connections between overworked synapses, and that sleep helps consolidate and re-form important memories. In this review, we will summarize these theories, but also focus on substantial new information regarding the relation of electrical brain rhythms to sleep. In particular, while REM sleep may contribute to the homeostatic weakening of overactive synapses, a prominent and transient oscillatory rhythm called “sharp-wave ripple” seems to allow for consolidation of behaviorally relevant memories across many structures of the brain. We propose that a theory of sleep involving the division of labor between two states of sleep–REM and non-REM, the latter of which has an abundance of ripple electrical activity–might allow for a fusion of the two main sleep theories. This theory then postulates that sleep performs a combination of consolidation and homeostasis that promotes optimal knowledge retention as well as optimal waking brain function.
Sleep is clearly a basic human drive, yet we do not fully understand its purpose or function. One could argue that quiet but conscious rest could be just as efficient as sleep for recuperating certain parts of the body and would be less dangerous, since the brain would not be closed to outside inputs. From the evolutionary point of view, then, unconscious sleep must offer an unseen advantage to the brain.
In attempting to understand the neural implications of sleep and neural activity during sleep, the field has focused on the view–well supported by data –that sleep benefits memory and general neural function. In more recent years this claim has been split into two subdomains: 1) a hypothesis centered on homeostasis, wherein sleep reverses the over-elaboration and exhaustion of neural networks brought about by prolonged waking states; and 2) a hypothesis that sleep consolidates important memories for long-term storage. In sleep theory, as in neuroscience, much attention has recently been focused on synaptic connections, which carry information between neurons. Yet at the level of the synapse, these two theories seem to conflict: while the homeostatic theory states that synapses, in general, are weakened, the consolidation theory states that selected synaptic connections should be strengthened during sleep as a way to consolidate memory.
We seek here to summarize the major concepts in the neuroscience of sleep (and refer the interested reader to a more comprehensive review of the relationship between sleep and memory).1 We propose that electrical brain rhythms are key physiological features that allow the brain to carry out all aspects of the tasks of sleep and that offer important insight into those tasks. We also seek to determine whether these two apparently opposing views on sleep might be reconciled. . . .
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- 1Björn Rasch and Jan Born, “About Sleep’s Role in Memory,” Physiological Reviews 93 (2013): 681–766.