It's not advisable if you listen to CW, no. But as you've probably guessed by now, the vast majority of CW wisdom is bollocks. If you wake up hungry, by all means have breakfast. I did today. It'll probably be a week or so before I have breakfast again (or rather, I have breakfast combined with lunch - a large meal around 2pm, then dinner around 7-8pm). I'm not dead or malnourished yet.