When you workout and and undertake High Intensity Interval (HIIT) training, your core body temperature rises because your body is naturally inefficient. To produce 300 watts of power, it takes about 1000 watts - this leaves 700 watts which is released primarily as heat energy.
As the core body temperatures rises and you get hot, more blood is diverted from the muscles (for power generation) and to the skin for cooling. Sweat evaporating helps cool your skin and your blood and enabled your body to cool. This could be considered a limitation of the human body because the fuelling system is connected directly with the cooling system.
At the end of a workout with interval training, the intervals at the end of the session are (or should be) harder than the intervals at the start of the session. Fatigue is an obvious cause though the connection with a rising core body temperature is usually not considered or is overlooked.
If you analyse a HIIT workout, the threshold power will commonly drop 20% when the core body temperature is elevated. When you get hot, it is harder to do intensity efforts.
Heat Compensation / HIIT Interval Power Compensation
Considering the relationship between core body temperature on training, this opens up the scope to compensate interval training - HIIT Interval Power Compensation.
An example would be starting interval training well above FTP and slowly lowering this FTP and finishing below FTP to compensate for the rising core body temperature. The principle is to establish an equilibrium in which the effort... how hard the training feels... remains constant.
Of course, achieving this result means testing and adapting training to suit your abilities and goals. While core body temperature adjusted pacing already being employed by elite athletes, the principles also transfer into training and can add a new dimension to interval training.