Haruki Murakami, the excellent Japanese novelist, wrote about the sign inside his Japanese gym. It was something like,
"Fat is easy to get and hard to lose. Muscle is hard to get and easy to lose. Such is life."
It's similar to another Japanese expression about money --
"Getting money is like digging in sand with a needle, spending it is like water soaking into sand."
I remember where I read the first quote -- it was reading "What I Talk About When I Talk About Running" in 2008, when I was in Barcelona and living near Placa Espania. At the time, I happened to be on a fantastic routine of waking up at 4:30AM, reading and eating in a little Spanish shop that served a fantastic Spanish egg/potato/vegetable omlette until my gym opened, spending a couple hours in the gym, and then working until dinner before sleeping early.
I set Nap as a required daily activity a bit back.
It's been marvelously amazing.
The first few hours after waking, in general, are just so much better for willpower and creativity.
A decent nap -- sometimes even just 20 minutes will do, sometimes longer -- seems to give 90% of the benefits of "just woke up for the first time.
It's really marvelous. Try it. I'm mandating it for myself under normal circumstances, rearranging my schedule to nap, etc. It's that good.