Arsenal defender David Luiz admits former manager Unai Emery struggled to inject "fun" into training.
Emery made a promising start when he first took the reins at the Emirates, but started this season poorly after being bankrolled by the board in the summer window.
On how Arteta has restored the positivity at Arsenal, Luiz told the club's official website: "If you sleep four hours happy, you're going to sleep better than eight hours sad.
"It's like I said in that interview, and I stand by it. The result of people when they are happy is 10 times better than when they are sad.
For me, you have to have fun not just in football, but in life. That's the most important thing – and Mikel Arteta has brought that back.
"Working with Mikel has been great since the first day. He's always working straight to the point and he has already shown us his ideas, his goals. He wants to try to bring the shine back to this club with a winning mentality and by working hard every single day.
"He's trying to do everything as well as possible to achieve that."