The skin is the largest organ in the human body. It protects the body from the sun's rays and has a vital role to play in thermoregulation, keeping the body temperature within normal ranges (circa 37 °C).
Skin temperature depends largely on air temperature and time spent in that environment. Such weather factors as wind chill and humidity cause changes in skin temperature. Skin temperature oscillates predictably with the time of day. Thus, a healthy temperature at any site is described by a range of values, rather than a fixed value. This is important because when temperature is used to determine the relative health of an individual, a group mean will not necessarily accurately categorize an ideal temperature for all members of that group. Some healthy individuals would be expected to have values higher or lower than the mean and these values would, in theory, be normal for them.
Time-of-day difference is critically important when monitoring temperature since it introduces a large and consistent source of variation. Available findings indicate that body temperature predictably reaches its lowest value during the three hours prior to waking (03:00-06:00) and has a high value approximately 12-14 hours after waking. Failure to account for this healthy diurnal variation in temperature can lead to an erroneous conclusion that an individual's temperature, uncorrected for time of day, suggests a disease state when it is in fact a healthy temperature at that time of day.
Rather than any one temperature value in isolation, the degree of change of temperature over 24 hours might be a more important determinant of health in a given individual, with greater difference being consistent with better health. Two factors that can influence the degree of change are a person's relative physical fitness and age. In existing studies, greater fitness and younger age are characterized by larger temperature amplitudes, while poor fitness and advanced age are characterized by diminished amplitude.