Human adaptation to repeated short term exposure to cold appears to be characterised by a decreased shivering threshold and unchanged sweating threshold, producing a widening of the inter-threshold zone (1). As a consequence, deep body temperature may fall more rapidly in cold habituated individuals on exposure to cold. This 'hypothermic adaptation' (2) may contribute to a range of problems, from hypothermia in the elderly to insidious hypothermia in occupational groups such as divers.
Although a reduction in the metabolic response to cold is probably the most widely and frequently reported alteration with cold habituation in humans (3), the nature of this attenuation remains unclear. It may be caused by a change in either the threshold for the onset of shivering, or in the sensitivity (gain) of the shivering response, or both. Furthermore, it may be specific to the body temperatures encountered during the habituation regime, or more generalised.