It should not be causing the CPU to overheat at 25% usage, the fan kicking in is not due to overheating, more-so just a feature of active cooling.
Saying that, it shouldn't be using 25%. It's plausible that whilst the PC is not being used, the processor underclocks (my i5 can be anywhere between 800MHz at idle, and 3.2GHz under load), and it's possible that the "25%" is referring to 25% of the capacity of the CPU whilst not under load.
How is it stopping the laptop going to sleep?