Sometimes while we are asleep, our subconscious (namely, our thoughts and desires, impulses and past experiences) are revealed unconsciously. We may be sick or hungry, or be facing an unsolvable problem. The imagination gives form to the deviations of a bad temper, or the mind remembers a past exciting event and gives it a new, different form. All such dreams are jumbled; they have some meaning, but are not worth interpreting. For example, if we eat salty things before sleep, we may dream that we are lying by a pool; if we go to bed angry, we may dream that we are fighting with others.
If we do not know how to interpret dreams, true dreams may be confused with or taken for such jumbled dreams. For example, although the dream Pharaoh told to Prophet Joseph, upon him be peace, was true, his men described it as jumbled. 
True Dreams. One type of dream has nothing to do with the subconscious self. Such dreams carry important messages: either good tidings from God, which encourage us to do good things and guide us, or warnings concerning the evil we have done. Those dreams, which we call true dreams, are very clear and unforgettable.
Some true dreams contain news of the future. To understand the nature and mechanism of such dreams, consider the following:
As a book's essence—its meaning—exists before it assumes a written, visible form, everything has an essential form of existence in God's Knowledge before it appears in the world. Islamic philosophers call these essential forms archetypes. When God wills to send them to this world, through the manifestation of His Wisdom and Power and the appropriate Divine Names, He clothes them in material bodies. Between the world of archetypes (where God's Knowledge has primary manifestation) and this world is another world—the world of immaterial forms or symbols. There, things exist in ideal forms or as symbols, and the concept and measure of time are completely different from their counterparts here. Dreamers find or receive these symbols differently, based on such factors as time and place, culture, and even national and individual characteristics.
When we sleep, our spirit ascends to this world of ideal forms without completely breaking its connection to the body.  It enters a different dimension of existence, where past, present, and future are combined. As a result, we may experience a past event or witness a future one. However, since things in that world exist in ideal forms or symbols, the spirit usually receives symbols that require interpretation.
For example, clear water there might correspond to knowledge here. If you see your own waste matter, it may be interpreted to mean that you will earn money in lawful ways; if the waste matter belongs to others, its may mean that money will come to you in unlawful ways. As mentioned in Sura Yusuf, a fat cow may mean a year of abundant crops, while a lean one means a year of severity. The metaphors, similes, and parables found in the Qur'an and the Prophetic sayings, and sometimes among people, may provide significant keys to dream interpretation. Some true dreams are so clear that no interpretation is needed.
As time is measured differently these two worlds, and as the spirit is far more active while we are dreaming, great saints who free their spirits, to a certain some degree, can travel long distances in a much shorter time than normal people.
Many people have had true dreams. For example:
• Abraham Lincoln's dream the night before his assassination is famous. He dreamed of White House servants running to and fro, telling each other that Mr. Lincoln had been killed. He woke up in great excitement and spent an uneasy day. Despite warnings, he went to a theater that evening and was killed.
• Eisenhower's dream just before he landed on Normandy in June 1944 changed the course of the Second World War. A few days before the date on which he had decided to land, he dreamed that a big storm broke out and overturned the landing crafts. This caused him to move up the date. History records that his dream was accurate.
• The mother of Anne Ostrovosky, a Russian writer, saw many scenes of the German–Russian battles 5 years before the Second World War broke out. Her dream was published in several newspapers.
• Several scientific or technological discoveries were first seen in dreams. Elias Howe, while trying to figure out how to thread a sewing machine, dreamed he was a prisoner of an African tribe that wanted him to thread a sewing machine. In mortal fear and puzzled, he suddenly saw holes at the ends of his captors' spears. He woke up and made a little "spear" with a hole at one end. Niels Bohr, who was studying atomic structures, dreamed of planets connected to the sun with threads and turning around it. When he woke up, he conceived of a resemblance between what he had dreamed and atomic structures.
Many other true dreams have predicted future events or resulted in scientific or technological discoveries. But these examples are sufficient to show that dreams are the result of the sprit's journey in inner dimensions of existence (the world of immaterial forms or symbols) and of receiving signals therein.
Finally, dreams provide a strong proof for the existence of immaterial worlds as well as for Divine Knowledge and Destiny. If God Almighty had not predetermined and recorded all events in "the Supreme Guarded Tablet," how could we be informed of future events? Also, dreams show that the measure of time differs greatly according to each world's features.
 During dreams, the spirit continues this connection through a cord.