SB 11.25.18
sidac cittam viliyeta
cetaso grahane ’ksamam
mano nastam tamo glanis
tamas tad upadharaya
Translation:
When one’s higher awareness fails and finally disappears and one is thus unable to concentrate his attention, his mind is ruined and manifests ignorance and depression. You should understand this situation to be the predominance of the mode of ignorance.