Read the following verse deeply.
"Mind consciousness is always functioning
except in states of non-perception,
the two attainments,
deep sleep, and fainting or coma."
Thich Nhat Hanh (Thay) showed the above verse and explained more clearly about the five circumstances under which mind consciousness ceases to function. The followings are those five circumstances:
1) meditative concentration (samadhi)
2) meditative states of no thinking (one of the two attainments)
3) meditative states of no perception (one of the two attainments)
4) deep sleep without dreaming
5) while in a coma or in a faint
Deep sleep is one of the five circumstances. Thay mentioned, "In a state of deep sleep there are no conscious dreams. When we dream, it means that our mind consciousness is active. In deep sleep, we can naturally go into the state of no-mind consciousness. Dreamless sleep is very refreshing and revitalizing because our mind consciousness, which is usually so active, is able to rest."
I understand that we (brain) stop thinking and awareness revives during the deep sleep state. Anyway, deep sleep has exactly the same effects of meditative concentration (samadhi), meditative states of no thinking and no perception. In other words, this means that every human meditates unconsciously during the deep sleep state. Meditation brings about effects such as stopping, resting, calming, healing, nourishing. Therefore, it is needless to say that the deep sleep plays an important role in human life.
Thay said, "suffering often arises because of what we see and hear and the mental formations that arise based on our perceptions." So, non-perception can stop the arising of the unwholesome mental formations, or afflictions. In this sense, we can never suffer in our deep sleep state. So, meditation can be said as a deep sleep without dreaming while awake.
(note) perception: series of notions in the head (brain)