As human beings, we need life energy to function.
During our daily functioning, we consume life energy.
During sleep, our soul ensures that the used life energy is replenished. This only takes place when we have entered deep sleep. When the life energy is replenished we feel rested and fit enough to be active during the day.
-You feel sleep close to you, it is tangible; and yet, do you know your sleep? The smallest insect possesses this wonder, this peace, in order to regain new strength, but it is a great mystery. You can fall asleep every moment of the day, yet you know nothing as a human being about this incredibly deep divine wonder, which is your sleep. Science does not know your sleep either. The academics are still faced with the great problem and do not know what to do about it. Only the East has learned to know sleep.
Spiritual Gifts p.390,391 (SP.9636.9640)
–The healthy human being, finding himself in the fourth grade of sleep, knows nothing more about this restless tossing about. The nervous system has been disengaged and can no longer react, at least not at full power, because it remains in operation, or death would enter. The soul life is also completely separate from the organism. The soul now absorbs the powers for the organism, draws those powers to itself from the Divine universe and passes them on to the organism during sleep. It is only possible to gain strength in the fourth grade; the third grade is still consciously materially tuned in. The fourth grade is the uncon- scious falling asleep and then the soul life has entered that phase. Now the soul can absorb the living and vital astral aura.
Spiritual Gifts p.393 (SP.9636.9640)
-Do you see, my sisters and brothers, do you now understand that the sleep – we will soon come to those philosophical systems, remember this – that your sleep is precisely: the going from the material back to the spirit?
Lectures3 p.64,65 (L3.1658)
Source: Quotations from the books of Jozef Rulof