Because of the level of variation in temperature between people, there are no 'normal' temperatures - rather there are average and typical temperatures which depend on the activity.
A typical resting core body temperature range for a human being is between 36.1°C / 97°F and 37.2°C / 99°F.
During the day, the temperature fluctuates and this is known as the Circadian Rhythm so the widely regarded temperature of 36.9ºC / 98.4ºF is a daily average and subject to variation. While sleeping the temperature will drop slightly and during the day it will rise.
During sport, the core body temperature can increase above 38ºC / 100.4ºF
Temperatures above 39ºC / 102ºF start to put the human body at risk of heat-stress although the training and conditioning of each individual means that the tolerance varies significantly. Our observations of elite athletes have shown top thresholds in competition range between 38.5ºC - 41ºC / 101.3ºF - 105.8ºF and suggest that athletes who monitor their own core body temperature will have a significant advantage in training and competition.
As a generalization, an athletic / trained person commonly operate better when hotter however optimal temperature and thresholds can still vary substantially.
What is a reliable temperature measurement?
Different thermometer methods give different core body temperature readings.
Rectal thermometers, ingested e-pills and bladder and heart catheters are all invasive (internal) thermometers which are also considered to be accurate and hence are suitable for medical use and for lab testing.
Laser scanners, ear thermometers, oral thermometers and other skin temperature calculators are considered less accurate and less consistent so the measurement values delivered by a single device or in comparison to other thermometers can vary significantly. Temperature also varies on different parts of the body (for example, the extremities such as arms and legs can be cooler.
The CORE sensor is has been tested extensively against rectal thermometers and e-pills as 'controls' to provide a similar level of accuracy. For lab testing with controlled conditions and environments, the internal thermometers are favoured while for field and practical scenarios such as training and competition, the CORE sensor is convenient and discrete and provides a suitable high level of accuracy.