His method is fine, but it's not the only one, not by a long shot.
Tons of successful people, Benjamin Franklin, Joe Girard, top CEOs, and more plan(ned) in the morning.
Jack Canfield's point about the subconscious mind working while you sleep cuts both ways. On one hand, sure, you could be thinking about your plan... perhaps made late at night while tired. Or earlier in the evening if you did it then.
Or... you can think about your life, read to learn and retain new information, socialize, etc. in the evening... dream about these things all night... and wake up refreshed, making your plans after having the benefit of your thinking.
Plenty of people have "slept" on their problems knowing full well they'd be in a better frame of mind, more energetic, and having given it more thought, to have an answer in the morning.
I'm not saying Canfield is wrong... but it's hardly the only way.