Consider man while in the state of sleep; it is evident that all his parts and members are at a standstill, are functionless. His eye does not see, his ear does not hear, his feet and hands are motionless; but, nevertheless, he does see in the world of dreams, he does hear, he speaks, he walks, he may even fly in an airplane. Therefore, it becomes evident that though the body be dead, yet the spirit is alive and permanent. Nay, the perceptions may be keener when man’s body is asleep, the flight may be higher, the hearing may be more acute; all the functions are there, and yet the body is at a standstill. Hence, it is proof that there is a spirit in the man, and in this spirit there is no distinction as to whether the body be asleep or absolutely dead and dependent.
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 242)
In the time of sleep this body is as though dead; it does not see nor hear, it does not feel, it has no consciousness, no perception: that is to say, the powers of man have become inactive, but the spirit lives and subsists. Nay, its penetration is increased, its flight is higher, and its intelligence is greater.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 327)
Many times when Shoghi Effendi was intensely distressed, I saw him go to bed, refusing to eat or drink, refusing to talk, rolled under his covers, unable to do anything but agonize, like someone beaten to the ground by heavy rain; this condition sometimes lasted for days, until forces within himself would adjust the balance and set him on his feet again. He would be lost in a world of his own where no one could follow.
(Rúhíyyih Rabbani, The Priceless Pearl, p. 45).
Must forget his own selfish conditions that he may thus arise to the station of sacrifice. It should be to such a degree that if he sleep, it should not be for pleasure, but to rest the body in order to do better, to speak better, to explain more beautifully, to serve the servants of God and to prove the truths. When he remains awake, he should seek to be attentive, serve the Cause of God and sacrifice his own stations for those of God. When he attains to this station, the confirmations of the Holy Spirit will surely reach him, and man with this power can withstand all who inhabit the earth.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 384)
O Son of Man! Many a day hath passed over thee whilst thou hast busied thyself with thy fancies and idle imaginings. How long art thou to slumber on thy bed? Lift up thy head from slumber, for the Sun hath risen to the zenith, haply it may shine upon thee with the light of beauty.
(Bahá’u’lláh, Hidden Words, Arabic 62).
O Son of Man! Ponder and reflect. Is it thy wish to die upon thy bed, or to shed thy life-blood on the dust, a martyr in My path, and so become the manifestation of My command and the revealer of My light in the highest paradise? Judge thou aright, O servant!
(Bahá’u’lláh, Hidden Words, Arabic 46).
… Our body is like the cage, and the spirit is like the bird. We see that without the cage this bird flies in the world of sleep; therefore, if the cage becomes broken, the bird will continue to exist: its feelings will be even more powerful, its perception greater, and its happiness increased. In truth, from hell it reaches a paradise of delights, because for the thankful birds there is no paradise greater than freedom from the cage.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 327).
Regarding your question: there are very few people who can get along without eight hours sleep. If you are not one of those, you should protect your health by sleeping enough. The Guardian himself finds that it impairs his working capacity if he does not try and get a minimum of seven or eight hours.
(Shoghi Effendi, Compilation of Compilations, V I, p. 459-488)
What result is forthcoming from material rest, tranquillity, luxury and attachment to this corporeal world! It is evident that the man who pursues these things will in the end become afflicted with regret and loss.
(‘Abdu’l-Bahá, Tablets of the Divine Plan, p. 42)