If cereal is your go-to breakfast, you may be lowering your energy levels before your day has even started.

Eating cereals that are high in sugar, and other simple carbohydrates like white bread that immediately break down into sugar, will lead to an instant spike in your blood sugar, according to Dr. Nancy Rahnama, an internist and clinical nutritionist. 

"And anything that spikes is going to crash," Rahnama tells CNBC Make It. "This high-end crash and burn throughout the day leads to fatigue, irritability, changes in the mood, headaches, hunger [and] cravings."

High-glycemic carbohydrates, also known as simple carbs, can affect how refreshed and alert you feel in the morning, too, if you eat them for breakfast, says Raphael Vallat, a postdoctoral researcher at the Center for Human Sleep Science at U.C. Berkeley.

But, these foods aren't limited to just cereal; potatoes, white rice, white toast, muffins and donuts can have the same effect, the experts say.

Instead, you want to aim for low-glycemic, complex carbohydrates and foods high in fiber for breakfast, according to Rahnama.

Breakfast foods that will keep you energized

The complex carbs that Rahnama and Vallat recommend for breakfast, instead of cereal, white toast and muffins, include:

• Whole bread (consider adding nut butter)
• Whole grain oats/overnight oats
• Protein shakes (add chia seeds for extra protein)
• Eggs
• Brown rice

"I never recommend cereal to my patients to have for breakfast, simply because it's processed. I don't recommend anything that's processed," says Rahnama.

Alternatives to try if you enjoy cereal for breakfast are non-processed overnight oats or granola, she adds.

Additionally, if cereal is all you have time for, aim for ones that are high in fiber and low in sugar.

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