Sleep-promoted memory consolidation is well known: several studies demonstrated how sleep might improve the acquisition of new information (i.e., declarative memory) and of new motor skills, for instance when playing an instrument or practicing sport (i.e., procedural memory). In contrast, the relationship between sleep and a different type of memory, immunological memory, remains poorly understood. Although those memory systems differ in the quality of the encoded information, a study recently published by Besedovsky and colleagues illustrates an unexpected similarity.

Both in the central nervous system and in the immune system, memory formation consists of three phases: encoding, consolidation and recall. Encoding is the phase of the first contact with new information. For what concerns the immune system, information is represented by the molecules of infectious agents, i.e., the antigens. During an infection, the Antigen Presenting Cells (APCs) recognize, process, and present the antigens to the cells responsible for long-term immunological memory: the lymphocytes. Just like during sleep the information acquired during wakefulness is transferred in the brain areas responsible for long-term memory storage (consolidation), the antigen presentation is enhanced by sleep. At the end of the infection, lymphocytes remain quiescent in the lymph nodes and bone marrow, available for recall: in case of re-infection, memory lymphocytes will trigger a more rapid and efficient immune response. To sum up, sleep promotes immune system response by improving the consolidation of the immunological memory.

The clinical implications of this physiological process are particularly evident in vaccination studies. Vaccines contain molecules that have antigenic properties: without causing an infectious disease, they stimulate the immune system as a bacterium or a virus would, fostering the memorization of a new antigen, granting a quick and effective response in case of infection.