Basically, just eat less than you would a normal breakfast. The earlier you can eat it, the better, and the more you should consume.
Example (numbers based on what I eat, and will definitely differ by person, so take this as an example only):
- Breakfast 3 hours before a ride: 700+ calories (apple, banana, 2 servings oatmeal, some nuts)
- Breakfast 2 hours before a ride: ~500 calories (apple, banana, 2 servings oatmeal)
- Breakfast 1 hour before a ride: 350-400 calories (2 servings oatmeal, apple or banana)
You definitely need a base amount for good performance no matter how soon you work out ater waking up, but I find that it's a bad idea (for me) to consume 700 calories one hour before riding in the morning. However, make sure you're taking enough food with you to make up for the calories you're not consuming in the morning.
Long story short, taper your calorie intake based on how long you're awake before riding.