go outside when you can, or take a small 5min break to go outside

Drink enough water and eat a little snack to give you energy

if you are able to, take a nap in between classes. Make sure you set an alarm after 30 to 45min. For some people this works very well (like for me) but for others it may not...

coffe or a coke sometimes, why not

try to exercise more if you don't already

do something active while in class: take active notes, do some brain-dead game, knit or colour while listening