Sleep: Neural Optimization as an Ultimate Function for Memory Consolidation

Abstract

Many studies support the role of sleep in memory consolidation. The performances improve after sleep as compared to being awake for the same duration in the following night after learning. Besides, sleep-dependent memory consolidation does not seem to depend on specific states of sleep; instead, each stage of sleep contributes differently to the memory consolidation process. The process of memory consolidation involves many pieces of biological machinery, such as activation of second messenger systems, increased expression of memory-associated genes, induction of long-term potentiation and depression, the formation of new spines and new synapses, recruitment of new receptors, and generation of new neurons/circuitries. Most of the memories require the activation of these processes for offline memory consolidation during sleep. It is not known, however, how sleep triggers many types of biological machinery of memory consolidation. There is evidence that supports the role of sleep-related oscillatory waves in determining the timing of neural activity, which can, in turn, trigger and activate all necessary biological machinery essential for memory consolidation. In addition, data suggest that sleep plays a critical role in maintaining the optimal condition of the neural system to begin the processes of memory consolidation. Here, we put forward a “system optimization theory” (SOT), according to which a crucial function of sleep could be the maintenance of optimal neural conditions for the process of memory consolidation to take place. We propose “SOT” in an attempt to answer a very crucial question, “why sleep remains a favored state for memory consolidation?”