There is one world Tatva (base) please remember it. when person have basic knowledge of anything its called Tatva Gyan (Elementary knowledge). Which is become the principal knowledge. Like in science whatever you see are made from pure or compound of Elements; Elements are made of Particles Electron, Neutron and proton; hence whatever you see are made from Electron, proton and Neutron. (refer: periodic chart). Similarly for the Human they are do all karma. All Karma are done by three things; Mind, Body and Speech. Body do acts(karma) by 5 Gyanendriyas (organs of knowledge) and 5 Karmendriyas(organs of senses).
There is require to understand mind, beyond being mind. Mind have five states. These are 5 states of consciousness in human forms.
Jagruti - Waking
Swapna - Dreaming
Sushupti - Deep Sleep (slumber)
Turiya - Underlying state in all the above states
Unmani or Turyateeta - State of enlightened beings where the cosmos is experienced as oneness.
So when mind is working its jagruti, when we are sleeping their are two possibilities, one either deep sleep when we have no thoughts, that give rest to mind like body have rest without it working, Turiya is stage when Consciousness of mind is become more higher. Dhyan (Meditation) is stage when the mind is in jagruti(alive, awaken) but which is not thinking. Like we awaken but in rest our body is not making movement, the Consciousness is possible, when mind is awaken but its not thinking on anything which give great rest to the mind.
(1) Who goes for sleeping?
As per above answer you understand that sleeping is one of the five stage of consciousness of mind. sleeping is our mind is not working and no thoughts, dream is different stage of mind. No thoughts in mind is the deep sleep stage of the consciousness, and we feel rest of mind on second day.
(2) On which sleeping is depended?
Mind; It is said Man hi karta Man hi Bhokta, means Everything is done by mind, and Mind what feel. Like science whatever we see, smell, taste are determined by mind, mind is what make you feel, same way in Spirituality too, Mind is what feel each and everything. All Sences organs are different but the karma of them is feel by mind. so whatever you see, feel, taste, hear, speak is what the mind is doing.
(3) What does it means?
God has made us and given lots grace, if there was no sleeping, people could never rest in their life. All things appear is Maya(illusion). So by giving Nindra(sleep) we thanks to divine. Like we can thanks for water, rain, air, Human body. We thanks to divine and feel grace, that they give it to us. If you have bulb in your house and you think about the Edison, and think if there was not invented you immediate feel grace (thankfulness) to him, like we thanks for giving Nindra(sleeping) so we can feel rest of mind and freshness.
(4) Where does our body gets charged during sleep?
The Mind, If we run for 1 km or work for whole day we feel so much tired, and we feel to give rest to the body right, same way sleep is the consciousness of mind which give us the Rest. Meditation is the process which learn that in awaken (Jagruti) stage, we have to give rest to our mind, by not thinking on miscellaneous things. This is possible by the Meditation. The practice of which make you capable to avoid unnecessary thinking possible, and to give rest to mind by not thinking on anything.
The senses and Body can be rest even we are not sleeping, just sit in rest that's it. Jivatma is always witness of everything. and All what we acts are only mind, Hence all Karmendriyas(organs of action) and Gyanendriyas(organs of sense) are ultimately feel by mind.
