What causes people to feel sleepy due to recycled air?
Carbon dioxide is not the only waste product which is exhaled. NASA scientist Wolverton goes into detail in his book, "How to Grow Fresh Air".  We exhale 70% of the body's waste products in gaseous forms.  Breathing these in repeatedly and at ever-higher concentrations causes a whole host of health problems, starting with, you guessed it, an inability to stay alert and sustain full brain function.