The general idea is that sleepiness and sluggishness is a certain outcome of an unhealthy body, as well as anxiety and irritability that are caused by various body aches.
In the worst case, meditation can become a formality, just a checkbox to fill out, a way to spend an indulgent, lethargic half-hour, or the opposite - a constantly distracted, obsessive anxiety (which is perfectly fine and even to be expected at the beginning of a meditation, but not at all times).
That's not to say that meditation is completely off limit: the idea is to do a bit of meditation and breath exercises at first and a lot of asanas, and then gradually increase breath exercises and then concentration; and finally meditation.
But this whole idea that meditation can be counter-productive in some circumstances is not really alien to the Eastern traditions. They are not stupid and they've been doing it for a long time with an empirical, practical approach.