Yōga Sūtra 1.10

अभावप्रत्ययालम्बना वृत्तिर्निद्रा।

abhāvapratyayālambanā vr̥ttirnidrā।

[अभाव प्रत्यय अलम्बना वृत्तिर् निद्रा।
abhāva = shortage; pratyaya = of cause; alambanā = supported; vr̥ttir = activity; nidrā = is sleep;]

Absence of interest and lethargy is sleep (nidra).

Sleep is supported by absence of cause. When cause like interest and desire are absent (may be due to lethargy or tiredness), it results in sleep.