Question about circulated air in incubators

[HEChicken]

I'm currently building an incubator and am going to install a fan to circulate the air. My question is, does the fan need to run all the time or does it cycle on and off along with the heat?

 

[wilkey44]

on both my spring times in a box I leave the fans on all the time and the thermostat just turns on the heat lamps/or bulbs

 

[rebel yell]

run it all the time.