Neurobehavioral & Immunological Memory in relation to various stages of Sleep

Memory functions encompass three primary phases: encoding, consolidation, and retrieval.

In the encoding phase, which occurs in the waking brain, the perception of a stimulus leads to the creation of a fresh memory trace, which is initially quite vulnerable to external disruptions and the passage of time, often leading to forgetfulness. In the consolidation phase, which occurs in the sleeping brain, this fragile memory trace undergoes a gradual stabilization process, possibly involving multiple cycles of short and long-term consolidation, aimed at reinforcing and assimilating the memory into existing knowledge networks. Finally, in the retrieval phase, which again occurs in the waking brain, the stored memory is retrieved and recalled.

Evidence suggest that the effective reorganization of memory is particularly established during slow-wave-sleep (SWS) or non-rapid-eye-movement (NREM). The subsequent rapid eye movement (REM) sleep, may strengthening the reactivated and reorganized memory at a molecular and synaptic level.

Further studies discuss that memory processing during sleep is not uniform, and that declarative memory (episodic and semantic memory) enhancement occurs the most during SWS, whereas procedural memory (motor and perceptual skills) benefits mainly from REM.

More recently, research in this field has broadened its scope, indicating that sleep not only benefits memory in the neurobehavioral domain but also in the formation of immunological long-term memories. This suggests that the formation of long-term memories may be a general function of sleep. There are initial indications that sleep-dependent memory formation in both the immune and central nervous systems shares common mechanisms, particularly during SWS. Sleep appears to support the reorganization of memory representations in the immune system, where epitopic information is extracted from antigens and stored by T cells during the interaction with antigen-presenting cells.

Consequently, it appears that sleep and wakefulness are linked to separate and mutually exclusive methods of handling memory. Certain sleep stages promotes the consolidation of various types of memories, which is at odds with the effective encoding and retrieval of information, those of which are essential for responding to the demands of the waking state.

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