Among 9 jhanas which is deep sleep?

Alright I will answer this since we have ruled out the jhana that only leaves the realm of infinite space, infinite consciousness, nothingness, neither perception nor non perception and cessation of feeling and perception

But we need to rule out the realm of nothingness and lower because there still exists desire there while in deep sleep none of even desire exists

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And further, with the complete transcending of the dimension of the infinitude of consciousness, (perceiving,) ‘There is nothing,’ Sāriputta entered & remained in the dimension of nothingness. Whatever qualities there are in the dimension of nothingness—the perception of the dimension of nothingness, singleness of mind, contact, feeling, perception, intention, consciousness, desire, decision, persistence, mindfulness, equanimity, & attention—he ferreted them out one after another. Known to him they arose, known to him they became established, known to him they subsided. He discerned, ‘So this is how these qualities, not having been, come into play. Having been, they vanish.’ He remained unattracted & unrepelled with regard to those qualities, independent, detached, released, dissociated, with an awareness rid of barriers. He discerned that ‘There is a further escape,’ and pursuing it, he confirmed that ‘There is.’

https://www.dhammatalks.org/suttas/MN/MN111.html

We also need to rule out cessation of perception and feeling since a person of no integrity can't enter that while they can still enter deep sleep

And further, a person of no integrity… enters & remains in the second jhāna… the third jhāna… the fourth jhāna… the dimension of the infinitude of space… the dimension of the infinitude of consciousness… the dimension of nothingness… the dimension of neither perception nor non-perception. He notices, ‘I have gained the attainment of the dimension of neither perception nor non-perception, but these other monks have not gained the attainment of the dimension of neither perception nor non-perception.’ He exalts himself for the attainment of the dimension of neither perception nor non-perception and disparages others. This is the quality of a person of no integrity.

“A person of integrity notices, ‘The Blessed One has spoken of non-fashioning even with regard to the attainment of the dimension of neither perception nor non-perception, for by whatever means they suppose it, it becomes otherwise from that.’ So, giving priority to non-fashioning, he neither exalts himself for the attainment of the dimension of neither perception nor non-perception nor disparages others. This is the quality of a person of integrity.

“A person of integrity, completely transcending the dimension of neither perception nor non-perception, enters & remains in the cessation of perception & feeling. When he sees with discernment, his effluents are ended. This is a monk who does not suppose anything, does not suppose anywhere, does not suppose in any way.”

That leaves on the realm of neither perception nor non perception as deep sleep which means every single day we enter the realm of neither perception nor non perception without we even realize every single day we do deep samatha meditation without we even realize

https://www.dhammatalks.org/suttas/MN/MN113.html

Jhanas are 8 distinct states of concentration that build the basis for insight practice and ultimately Enlightenment.

These 8 jhanas/dhyanas/samadhi do not encompass all the different states of concentration possible, just the key ones conducive to Enlightenment.

So yes, sleep is a type of concentration/altered state and even has some factors shared with the factors of jhana (like vicara, vitarka, bliss, joy) but still has present negative hindrances (torpor, unmindfulness).

In fact, this leads teachers to instruct us that if one is feeling extremely sleepy one should just go to sleep and take care of it... instead of struggling to meditate and cultivate jhana.... because the torpor and unmindfulness will intermingle and condition one's jhana training (damaging and hindering one's cultivation).

Interestingly, sleep itself is an altered state and belongs within the 4th skandha as one of the "Four Variable Events". This is more described in Abhidamma and consciousness-school probably but here is an interesting excerpt regarding this from "How to Measure and Deepen Your Spiritual Realization" a book that goes in detail regarding the skandhas.

The 2nd page regarding it may actually address your root question (without the incorrect idea that sleep is a jhana): "why is deep sleep spiritually beneficial"?

Jhana is meditation, not sleep (including deep sleep).

Sleep (including deep sleep) is not jhana.

In meditation (including jhana states), one is conscious and aware.

In sleep (including deep sleep), one is not conscious and not aware.

If you're not conscious and not aware, then you're not in meditation.

Then connection between sleeping and meditation is well explored in AN7.61. In this sutta, the Buddha advises Venerable Moggallāna on ways to avoid drowsiness, since drowsiness is hard to give up.

AN7.61:2.1: “Are you nodding off, Moggallāna? Are you nodding off?”
AN7.61:2.2: “Yes, sir.”
AN7.61:2.3: “So, Moggallāna, don’t focus on or cultivate the perception that you were meditating on when you fell drowsy. It’s possible that you’ll give up drowsiness in this way.

When you sleep, just sleep. When you meditate, just meditate.

NOTE: Deep meditation can reduce the need for sleep.

I've got a simple answer for you: Sleep is not a jhāna. The Buddha never says, not in Pāli not in Chinese not in Sanskrit, that sleep is a jhāna. Sleep is also not nibbāna. Sleep is also not nirodha. You just forget your dreams when you wake up and it seems like you were up to "nothing," but your mind was quite active despite you not remembering it.