Lethargy/sleepiness (昏沉) and hyper-activeness/lack of concentration (散亂) had been mentioned as the two ills in most classic Taoist texts. Similarly, in zhan zhuang (standing meditation), a practitioner is warned not to do "depression stance", yet not to look "too sharp" (i.e. should have a "stupefied" look instead of a "sharp" look).
In the beginning, an experienced coach usually can give good advice to a practitioner judging from his external appearance. However, in intermediate or advanced level, a student can actually fool himself and his coach, i.e. with his mind wondering away or in a sleeping mode without his coach able to notice him being so! On this matter, the grandmaster of West sect Li HanXu (李涵虛) had this to say in chapter one of his important classic "A study of Taoist aperture" (道竅談), he was talking about folded leg meditation.
If you avoid the light and eager to sleep, I can't find out; you must control yourself.
If you move occasionally and pretend to be awake all the time, I can't judge the difference, you must use your own will power.
If you close your eyes but thinking or worrying about different things, how I can konw, you must drive away your worries by yourself.
When you meditate with lethargy, you are not diligent enough.
when you meditate without concentration, you are not focused enough.
Not diligent and not focused will disappoint the good intention and expectation of our founding Grandmaster.