O Bhārata, know that tamo-guṇa, deluding all alike, is born of ignorance. It binds the embodied beings through inattention, lethargy and sleep.
Sattva-guṇa provides happiness and knowledge. Rajo-guṇa enables activities. The cycle of life is not complete without tamo-guṇa. The three together constitute complex Nature, governing the world and the beings within it.
The role of tamo-guṇa is to supplement sattva and rajas, by providing the antidote for both. If sattva instils knowledge and happiness, tamo-guṇa brings recession from these, by generating lethargy, sleep and ignorance. Lethargy and sleep intercept the stream of activity. Hence none can remain active for long. Every one needs a period of rest and sleep, a counter to wakefulness.
What a complementary role is that of tamo-guṇa! As hunger enables tasty food and appeasement, fatigue and subsequent rest facilitate recuperation and further activity. Like other two guṇas, tamo-guṇa also binds. It binds by undue clinging to sleep, rest and inattention. The three guṇas together make life, activity and experience.