When yīn is vacuous, there is internal heat

From Plain Questions (素问 sù wèn, tiáo jīng lùn).

1. (Originally) taxation fatigue damaging the spleen, causing spleen vacuity heat effusion.

2. (Nowadays) depletion of yīn humor gives rise to internal heat characterized by tidal heat, or vexing heat in the five hearts, as well as night sweating, dry mouth, red tongue, and a rapid fine pulse.