I have a self-employeed author who produces novels every couple days. I keep him working almost constantly.
Yes, his fun meter drops, but here's what I've done to mitigate that...
His workspace is beautifully decorated, his chair is super-comfy, he works on an upgraded laptop and has his favorite music playing. Before he starts to write, he takes a bubble bath (with his ducky), eats a perfect meal with his fiance' and plays computer games until his fun is maxed.
His fun meter will drop, but not too fast and he can generally finish 300+ pages before I need to give him any attention at all.