We are to know, love and serve God in this life and to share in the paradise of his Beatific Vision in the next life (CCC 1721). We know God only indistinctly in this life (1 Cor 13:12).
The fruition of sanctifying grace is the Beatific Vision, which is the theological term for seeing God face to face, i.e. as he knows and loves within the Trinity. In this mortal life no one can see God and live (Ex 33:20). Once enjoying the ecstasy of seeing God, no one could cease to possess the Beatific Vision, which is heaven, and then return to living this mortal life.
Although Moses talked to God as one man talks to another face to face (Ex 33:11), he did not see God as he is. At Moses’ request to see his face, God denied the request, but did permit Moses to see his back (Ex 33:23), a partial vision of God’s glory, perhaps like that of the transfiguration.
Although stories are told of a glimpse of heaven in what are called near death experiences, these alleged visions cannot be an experience of heaven, i.e. the Beatific Vision. Although we possess sanctifying grace in this life, we cannot experience its fruition, seeing God, and then continue to live or return to living this life.
My favorite premonition regarding heaven was that given to a dying Huron in a dream as told by St. Jean DeBrebuf in The Huron Relation of 1635:
Another good old man, having fallen sick, would not hear of going to heaven, saying he wanted to go with his ancestors. A few days later, he gave in, telling me an amusing tale: “Rejoice’, he said, ‘for I have returned from the land of souls and I found no one there. They have all gone to heaven.”
This private revelation to the dying Huron regarding heaven is of little wonder when we read of the Hypostatic Union through which Jesus alone could enjoy the Beatific Vision in his humanity while living this mortal life. In the encyclical, Mystii Corporis, June, 1943, Pius XII states:
75. Now the only-begotten Son of God embraced us in His infinite knowledge and undying love even before the world began. And that He might give a visible and exceedingly beautiful expression to this love, He assumed our nature in hypostatic union: hence—as Maximus of Turin with a certain unaffected simplicity remarks—”in Christ our own flesh loves us”.  But the knowledge and love of our Divine Redeemer, of which we were the object from the first moment of His Incarnation, exceed all the human intellect can hope to grasp. For hardly was He conceived in the womb of the Mother of God, when He began to enjoy the beatific vision, and in that vision all the members of His Mystical Body were continually and unceasingly present to Him, and He embraced them with His redeeming love. O marvelous condescension of divine love for us! O inestimable dispensation of boundless charity. In the crib, on the Cross, in the unending glory of the Father, Christ has all the members of the Church present before Him and united to Him in a much clearer and more loving manner than that of a mother who clasps her child to her breast, or than that with which a man knows and loves himself.
Unlike us, whose actions of the soul, namely knowing and loving, are completely dependent upon on our bodies in this life, Jesus was fully conscious in his humanity through the Beatific Vision when his body was developing in his mother’s womb and when he was asleep after birth.
Not so with us in this life. When we are asleep our intellect and will are inactive or only slightly active in dreaming. Death occurs when the material condition of our body is such that our soul can no longer animate it. After death, before we are purged of imperfection and see God in the Beatific Vision, we would naturally be completely unconscious except for some light granted by God to our souls, temporarily separated from our bodies.
Our human knowledge in this life is not the measure of the human knowledge of Jesus who fully possessed the consciousness, knowledge and love of the Beatific Vision in his human soul throughout the period of his mortal life from conception to death in waking and in sleeping.