यदग्रे चानुबन्धे च सुखं मोहनमात्मनः ।
निद्रालस्यप्रमादोत्थं तत्तामसमुदाहृतम् ॥ १८.३९ ॥

yad-agre cānubandhe ca sukham mohanam-ātmana: |
nidrālasya-pramādottham tat-tāmasam-udāhṛtam || 18.39 ||

Tāmasika delight

The delight that proceeds from and results in delusion, and is born of sleep, lethargy and inattention, is defined as tāmasika.

Tāmasika sukham is such that it keeps the individual under delusion throughout. Under its influence, one becomes averse to wisdom, knowledge. Inducing sleep, lethargy and inattention, tāmasika sukham works its havoc, permitting no spiritual growth, purity or refinement. It dupes spiritual seekers, by making them addicted to such practices and expediencies that lead to laya, stupor. Some under its spell remain absorbed, unconscious and inert for hours, even for months, contrary to the nature and purpose of spiritual life.

Seduced by it, some court jaḍa-samādhi, regarding it as the real goal of the spiritual pursuit. Drawing the mind to laya does not lead to true spiritual absorption within. Spiritual life must give rise to abundant alertness, fervour, efficiency, clarity, wisdom and all that these engender. Tāmasika sukham is fully opposed to this.

Krishna has already declared the spiritual effects of tamo-guṇa as ignorance and delusion. It binds one by inattention (pramāda), lethargy (ālasya) and sleep (nidra) (14.8,9 & 13). The same words find mention here too.