Allostasis and allostatic load are terms which supplement the classic terms homeostasis and stress. Allostasis is the active process that leads to adaptation to a stressor, and mediators of allostasis include stress hormones as well as the autonomic nervous systems and pro-inflammatory cytokines and metabolic hormones. Allostatic load and its more severe form, allostatic overload, represent the cumulative effects of chronic physiologic stress, which may be generated by internal processes (e.g., anxiety) and by external factors such as chronic stressors or by life styles (e.g., overeating, insufficient sleep) that also dysregulate the mediators of allostasis. Consequences of allostatic overload include many of the common diseases of modern life. In nature, however, allostatic load is used to achieve beneficial effects such as putting on fat for hibernation.