He comes with tales that could not be, Of wondrous visions eyes can’t see. His fables birthed from what you know, Yet when he leaves they too will go. What is he?

The answer to the riddle is sleep, a fundamental human experience that intertwines with our waking lives in mysterious ways. The "tales that could not be, of wondrous visions eyes can't see" refer to dreams, which are often vivid and fantastical narratives that unfold within our minds while we slumber. These "fables birthed from what you know" because our dreams frequently draw upon our daily experiences, memories, and emotions, weaving them into new, sometimes illogical, scenarios. Yet, when sleep "leaves, they too will go," as the vast majority of our dreams are quickly forgotten upon waking, with up to 95% slipping from memory within minutes.

During sleep, our brains remain remarkably active, particularly during Rapid Eye Movement (REM) sleep, which is when the most vivid dreams typically occur. While the exact purpose of dreaming is still a subject of scientific inquiry, theories suggest that dreams play a role in processing emotions, consolidating memories, and even rehearsing responses to potential situations. Our bodies cycle through different stages of sleep, with REM sleep becoming longer as the night progresses.

Ultimately, sleep is far more than just a period of rest; it's a vital biological process essential for our physical and mental well-being. It allows the body and brain to restore themselves, impacting everything from mood and memory to cognitive function and the immune system. Regulated by internal biological clocks known as circadian rhythms, sleep ensures we get the necessary time for these restorative processes, even if the fantastical stories of our dreams often vanish with the morning light.