Not A Good Breakfast
If you feel sleepy after your morning banana, you are feeling the effects of tryptophan. This is responsible for the after-turkey dinner sleepies.
Tryptophan slows down mental processes and reaction time. Add in the muscle relaxation from all the magnesium in bananas and you have a prescription for a great nap.
Try a banana at bedtime instead of on your morning cereal.
Make sure to brush our teeth afterwards, though. Bananas can be hard on your teeth.