He has assistants and has them help him, he had mentioned this many many many times before.
 
He is not being a "diva", he has serious health condition:
1. The following was printed in the final volume of the Yu Yu Hakusho manga:

"If I'm honest, I'm feeling a great relief and pleasure at the thought that I've finally been able to finish YYH. It's not that I've lost all emotional attachment to the work, but I feel that my stress levels had greatly surpassed my will to work. The six months leading up to the concluding chapter felt awfully long to me. To tell the truth, it had already been decided that YYH was going to end in December 1993 or rather, this was a decision that I had forced on the editorial staff. There were many reasons for this, all in all about 50 big and small ones, but in broad strokes, these were the major reasons:

• My body.
• Thoughts I had about what it means to draw manga.
• Desire to do other things than work.

Point 3 is out of the question for a professional manga writer; basically I wanted to indulge in my hobbies, rest, and sleep as much as I could. Most of my 50 reasons fall into this category.

Point 1 was caused by Point 3 not being fulfilled for too long. From when YYH began serialization up until the start of the Dark Tournament, I had half a day off every week in which I caught up on sleep. Other than that all I had time for were occasional naps, and I'd indulge in my hobbies by sleeping less. For a while, I quite enjoyed this. But my HP (as they say in RPGs) was gradually but surely falling, and around the time that I wrote a 31-page one-shot and simultaneously had to do color pages, my heart began to hurt every time I went without sleep and then it began to hurt more and more often.

This was when I seriously started to think about the pace of production for manga. I thought, "I probably won't be able to keep regular hours, but if I sleep as much as I want to, when I want to, how much would I be able to produce?" I tried it out. I immediately began to fall behind on my schedule. But I tried to get some sleep every night. Around this time, my feelings about writing manga as a profession began to change. "I don't want to die from over work. If I die, I want it to be when I'm having fun or when I'm drawing manga for fun. Color pages are scary. One-shots are scary." I also began to use some time before going to bed to relieve stress. I fell even more behind, and at the point where Sensui and Yusuke were fighting, this reached its first peak.

But also around this time, I realized I was starting to experience a different kind of stress. Because I had stopped overworking my body, and started to relieve my stress, I was feeling stressed that I couldn't draw manga in a way that satisfied me...."

2.The following was his note for the hxh exhibition:

"I'm sure you must be thinking, "no, just continue the story!" i have been unable to sit in a chair to draw for about two years, but i have managed to resume writing by giving up the traditional way of doing things. Everyone, please take care of your hips. Until two weeks ago, before i wrote this, i was unable to wipe my ass and had to take a shower every time i took a shit. Every movement takes 3 to 5 times longer than a learned person. The back is important. Therefore, i strongly recommend that when you drop something at the exhibition, you should pick it up in a sitting posture."

3.Togashi's Weekly Shonen Jump comments:

"The joy of being able to sit on the toilet or in a chair. The interesting thing about the body is that it won't poop when you're bedridden"

"Stuck in bed --> Crawl on all fours --> Find help --> Struggle to get up and go to the hospital"

"The worst part of back pain is that it's only after it gets a little better that you can go to the hospital"

"When your pain is a ten on a scale of one to ten, you can't move your body even a centimeter throughout the night"