In the Bhagavad-gita (8.17-18) it is stated that according to human calculations one day of Brahma is equal to one thousand ages of four millenniums (4,300,000 years) each, and the same period is calculated to be his night also. A Brahma lives for one hundred such years and then dies. A Brahma, who is generally a great devotee of the Lord, attains liberation after such a downfall. The universe (called the brahmanda, or the round football-like domain controlled by a Brahma) is thus annihilated, and thus the inhabitants of a particular planet, or of the whole universe, are also annihilated. Avyakta, mentioned here in this verse, means the night of Brahma, when partial annihilation takes place and the living entities of that particular brahmanda, up to the planets of Brahmaloka, along with the big oceans, etc., all repose in the belly of the virat-purusa. At the end of a Brahma's night, the creation again takes place, and the living entities, reserved within the belly of the Lord, are let loose to play their respective parts as if being awakened from a deep slumber. Since the living entities are never destroyed, the annihilation of the material world does not annihilate the existence of the living entities, but until liberation is attained one has to accept one material body after another, again and again. The human life is meant for making a solution to this repeated change of bodies and thereby attaining a place in the spiritual sky, where everything is eternal, blissful and full of knowledge. In other words, the subtle forms of the living entities take place in the heart of the Supreme Being, and such forms take tangible shape at the time of creation.