feeling worn out

year 12 student, 18 years old, male. not overweight, not anorexic (althought i'd like to gain weight/muscle)

i've been going to bed relatively early, eating well not missing meals (i used to miss lunch all the time, past few weeks i havent missed it) im not going through overly intense exercise, or anything to exhaust myself

however, i am just feeling worn out and sleepy alot of the time, got home and just sat down, arms feel heavy, almost feel like going to sleep, and just feeling meh. any ideas?