I sometimes feel sleepy during class, but why sometimes I feel awake all of the sudden?

Answer 1

An overload of carbs can do that to me. I will eat a big plate of spaghetti and then twnty minutes to an hour later I feel like sleeping. If I am out on the field running around then I feel great, but sitting in a classroom with all of those carbs ready to go will have the opposite effect.

Answer 2

Something is arousing your central nervous system, maybe a loud noise or tap on the shoulder...if you have sleep apnea, when your breathing obstructs, your oxygen level goes down, your CO2 level goes up, and this causes arousal that will wake you up.

Answer 3

changes in blood sugar levels do that.
You need more protien in your breakfast.