Bhagavad Gita · Chapter 14: The Three Qualities of Nature
Bhagavad Gita 14.13
अप्रकाशोऽप्रवृत्तिश्च प्रमादो मोह एव च ।
तमस्येतानि जायन्ते विवृद्धे कुरुनन्दन ॥
aprakāśo'pravṛttiśca pramādo moha eva ca | tamasyetāni jāyante vivṛddhe kurunandana ||
Dullness, inactivity, carelessness, and delusion: these arise when inertia has grown strong, Arjuna.
Modern context
The signs of tamas: dullness, inactivity, carelessness, confusion. These are the opposite of sattva's light and of rajas's energy. A tamasic mind is foggy and stuck. Everyone falls into it sometimes. The important thing is to recognise it for what it is, not to mistake it for peace. Real peace is clear and awake; tamas only looks like rest.