Andy Adams is dead on, again.
Being a good, mentally-engaged employee is tiring. Assuming your moonlighting work is in the same field as your day job, you’ll have already drained your mental “muscles” during the day.
At first, you’ll be ultra motivated. It will feel easy. The excitement of change brings a wave of energy to your after-hours work.
Give it a week or two. I’ll bet that, when you get home, you won’t feel like more coding/design/writing is what you want to spend your free time on. Your family, your hobbies, your life will start calling for your brain.
The rest of his post does a great job describing why moonlighting is so dangerous. I’d stop short of saying consistent moonlighting isn’t for anyone, but rather would say that it’s not for most people.
Moonlighting is hard, and takes a very special level of discipline to do well. I’ve never truly done it well. You probably haven’t either. Learn from Andy on how to transition effectively without burning yourself out moonlighting.