However, there are many variations in normal body temperature, and this needs to be considered when measuring fever. Body temperature normally fluctuates over the day, with the lowest levels at 4 ''[[A.M.]]'' and the highest at 6 ''[[P.M.]]''. Therefore, an oral temperature of 37.5 °C would strictly be a fever in the morning, but not in the afternoon. Normal body temperature may differ as much as 0.4 °C (0.7 °F) between individuals. In women, temperature differs at various points in the [[menstrual cycle]], and this can be used for [[family planning]] (although it is only one of the variables of temperature). Temperature is increased after meals, and psychological factors (like the first day in the hospital) also influence body temperature.
There are different locations where you can measure temperature, and these differ in temperature variability. [[Tympanic membrane]] [[thermometer]]s measure radiant heat energy from the tympanic membrane (=infrared). These may be very convenient, but may also show more variability.
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have decreased ability to generate body heat during fever, so even a low-grade fever can have serious underlying causes in [[geriatrics]].
In conclusion, temperature is ideally always measured the same moment of the day, in the same way, after the same amount of activity.
