When you ride on the road, there is always air moving over you causing the sweat to evaporate and keep you cool. There is no breeze when you are on rollers or a trainer, so you need a fan to keep air moving over you, as mentioned above. Something that may help you go longer is to put your rollers in a doorway. That way as you wobble your shoulders bounce off the door frame and you stay on the rollers. Doing that allowed me to be a lot more mindless while on my rollers.