Even downloading a file takes work by many mobo components. It is a low-load job compared to normal other tasks, but it still uses MUCH more energy (and hence heat generation) that doing nothing. So NO, it cannot be done in Sleep Mode. It CAN be done with low fan speeds because smaller amounts of heat are being generated.
Automatic fan speed control by your mobo will do that for you. HOWEVER such systems should never STOP any of those fans, because then there would have to be some extra efforts to re-start them when your workload increased again. Sleep Mode actually does stop them because there is almost NO heat to be removed in that Mode. But then, when the system exits Sleep Mode, it has a one-time re-start routine built into it for that single situation.