One verse

Srimad Bhagavatam 11.13.27

11.13.27

जाग्रत् स्वप्न: सुषुप्तं च गुणतो बुद्धिवृत्तय: । तासां विलक्षणो जीव: साक्षित्वेन विनिश्चित: ॥ २७ ॥

jāgrat svapnaḥ suṣuptaṁ ca guṇato buddhi-vṛttayaḥ tāsāṁ vilakṣaṇo jīvaḥ sākṣitvena viniścitaḥ

jāgrat
being awake
svapnaḥ
dreaming
su-suptam
deep sleep
ca
also
guṇataḥ
caused by the modes of nature
buddhi
of intelligence
vṛttayaḥ
the functions
tāsām
from such functions
vilakṣaṇaḥ
possessing different characteristics
jīvaḥ
the living entity
sākṣitvena
with the characteristic of being a witness
viniścitaḥ
is ascertained

Waking, sleeping and deep sleep are the three functions of the intelligence and are caused by the modes of material nature. The living entity within the body is ascertained to possess characteristics different from these three states and thus remains as a witness to them.