It is nutritionally based.
Banana is around 27g carbs (mostly fructose), negligible fat/protein. Yogurt is maybe another 20g carbs (sugar, lactose), maybe 8g protein?
So you're having around 47 g carb (mostly fructose and glucose) and 8 g protein for breakfast. No fat, no fiber.
The physiological result is increased serotonin levels which makes you sleepy. Due to the source of carbs, your blood glucose spikes but quickly goes back to zero.
You need to consume more protein and fats and limit the carbs. Some kind of meat/eggs and some kind of nut butter or normal butter. This is what will boost dopamine and acetylcholine, which are essential for motivation and focus. If you do have carbs, they need to be mostly fiber to sustain your blood glucose levels.
small steak, 2 eggs, spinach
unflavored greek yogurt with almonds in it