Human cold habituation: Physiology, timeline, and modifiers

Abstract

Habituation is an adaptation seen in many organisms, defined by a reduction in the response to repeated stimuli. Evolutionarily, habituation is thought to benefit the organism by allowing conservation of metabolic resources otherwise spent on sub-lethal provocations including repeated cold exposure. Hypermetabolic and/or insulative adaptations may occur after prolonged and severe cold exposures, resulting in enhanced cold defense mechanisms such as increased thermogenesis and peripheral vasoconstriction, respectively. Habituation occurs prior to these adaptations in response to short duration mild cold exposures, and, perhaps counterintuitively, elicits a reduction in cold defense mechanisms demonstrated through higher skin temperatures, attenuated shivering, and reduced cold sensations. These habituated responses likely serve to preserve peripheral tissue temperature and conserve energy during non-life threatening cold stress. The purpose of this review is to define habituation in general terms, present evidence for the response in non-human species, and provide an up-to-date, critical examination of past studies and the potential physiological mechanisms underlying human cold habituation. Our aim is to stimulate interest in this area of study and promote further experiments to understand this physiological adaptation.

Keywords: Adaptation; cold air exposure; cold shock response; cold water immersion; shivering; skin temperature; thermoregulation; vasoconstriction.

Figure 1.

Regulation of physiological thermoeffector responses to cold exposure. Decreases in mean skin temperature and core temperature are sensed by peripheral (skin) and central thermoreceptors. Cutaneous and central afferent signals are integrated in the preoptic area of the hypothalamus, which elicits insulative (heat-conserving) and metabolic (heat-generating) thermoeffector responses. Sympathetic signals descending from the pre-optic area mediate cutaneous vasoconstriction and non-shivering thermogenesis, while descending somatomotor signals activate shivering thermogenesis. POA, preoptic area; T_c, core temperature; T_sk, skin temperature.

Figure 2.

Representation of the thermal effector response (vasoconstriction, shivering) to a change in mean body temperature (ΔMBT) relationship. As mean body temperature decreases a thermal effector response (e.g., shivering) is elicited and increases (line A). The inflection point where this increase occurs is the threshold. The slope of the effector-ΔMBT relationship represents the sensitivity of the response. Line B denotes a response where the threshold is shifted, such that a thermal effector response does not occur until a larger ΔMBT occurs. In Line C, there is no threshold shift, but a change in the sensitivity of the response. For this example, line C denotes a greater sensitivity to a ΔMBT, that is, there is a greater effector for a given ΔMBT. Line D denotes both a threshold and sensitivity change. Reproduced from Castellani and Young, 2016 [2].

Figure 3.

Timeline of changes in perceptual and physiological responses due to cold habituation.

Figure 4.

Summary of the physiological and perceptual changes that occur due to cold habituation in humans.