When I was a kid, my classmates and I would frequently get temperature, feel tired or sleepy, and even have strong headaches during school days... ... usually

When I was a kid, my classmates and I would frequently get temperature, feel tired or sleepy, and even have strong headaches during school days... ... usually it happened just before a test, or a "long enough period of time" since our last such occurrence. Later on we found out going to the beach was a great help, but we never got teachers (or parents) to listen to us. :(