CCL13 and post-traumatic stress disorder: In healthy controls, the circadian clock program is intrinsically plastic, and changes, such as those induced by changes in metabolism, or sunrise time, are reversible.31 By contrast, chronic neuropsychiatric disorders have been associated with conditions in which the central clock appears to permanently ‘lose track of time'.32 Therefore, based on present data, the mechanisms linking central PTSD to individual peripheral circadian clock dysfunctions for MCP-1 or MCP-4 remain to be discovered.