Until about 10,000 years ago _ a tiny fraction of the physical evolution of our species _ we were a purely diurnal animal. The simplest and best strategy for all diurnal creatures is simply to hide away during darkness, avoiding predators and traps, and in any case not wasting energy when normal foods are hard to find anyway.
We assume that sleep is the primary biological adaptation to that fact. Yet our well-nourished brains remain active. Some science indicates that evolution then began to use the rest periods for other tasks such as memory cleanup or various housekeeping chores. Perhaps.
One result of all that is undoubtedly dreams. Dreams are quite difficult to deal with. They may seem to mean a lot or nothing. They may be gone, leaving nothing but an emotional residue, or they may remain as real as our waking memories. Some carry over from one night to another.
No wonder religions often address such visions. We all wonder if our subconscious is telling us something important, if our intuition has distilled meaning out of chaos, if the cosmos and gods themselves are speaking to us. In any case, dreams are at the least a grand immersive entertainment.
But we remain diurnal creatures, our “real world” is being awake. Seduction into believing those dreams are more important than our senses is one critical step into madness.