Bhagavad Gita Verse 8, Chapter 14

tamastvajnyaanajam viddhi mohanam sarvadehinaam |
pramaadaalasyanidraabhistannibadhnaati bhaarata || 8 ||
 
And, know tamas to be born of ignorance, deluding all the body dwellers. It binds them through heedlessness, laziness and sloth, O Bhaarata.
 
tamaha : tamas
tu : and
ajnyaanajam : born of ignorance
viddhi : know
mohanam : delude
sarvadehinaam : all body dwellers
pramaada : heedlessness
aalasya : laziness
nidraabhihi : through sloth
tat : it
nibadhnaati : binds
bhaarata : O Bhaarata
 
Tamas is our state of mind when it is dull and inert. Like a glass of dirty water cannot let light shine through, tamas prevents our mind from thinking clearly. Our mind starts to operate in moha, which means confusion or error, mistaking one thing for another. Tamas can be triggered by an overpowering emotional situation like Arjuna seeing his family members and loved ones on the opposite side of the battlefield. A tamasic state can also be triggered by abusing our sense organs through excessive drinking, smoking, watching TV and so on.
 
After a long and tiring day, when we want to rush to get a good night’s sleep, we get a call from a friend who wants to catch up. Try as we might, we will not be able to understand what he is saying because the mind has switched to a tamasic state due to exhaustion. Our memory does not work properly and our intellect’s logic is flawed. All we want to do is to rest our head on the pillow. When tamas reaches the height of its potency, we fall asleep.
 
Shri Krishna says that tamas is born out of ignorance. At its core, tamas keeps the Purusha, the jeeva, body dweller under the delusion that he is the body and not the self. When we forget our true nature as the self, the eternal essence, we assume that our body is who we are. This erroneous notion, this ignorance enables tamas to bind us, to trap us.
 
Tamas binds us in three ways, through heedlessness, laziness and sloth. Heedlessness is the performance of actions without intellectual focus or awareness. We either perform actions carelessly, such as dialing the wrong phone number, or perform actions that are futile, such as gambling or excessive alcohol consumption. Laziness is postponing or abstaining from our duties. Sloth is lying around in a state of stupor or sleep. Now, there is a place for resting and sleeping in our life, which is at the end of every day. But some of us derive joy from futile actions, from procrastination, from lazing around and so on. It is this joy through which tamas binds us.