I am currently reading about this book about DMT, it is a chemical in your brain that you give off when sleeping, also your body produces mass amounts of it when you die. Here is a paragraph from the book
"I already knew that the Tibetan Buddhist Book of the Dead teaches that it takes forty-nine days for the soul of the recently dead to "reincarnate." That is, seven weeks from the time of death of one person elapses until the life-force's "rebirth" into its next body. I remember very clearly, serveral years later, feeling the chill along my spine when, reading my textbook of human fetal development, I discovered this same forty-nine-day interval marking two landmark events in human embryo formation. It takes forty-nine days from conception for the first signs of the human pineal to appear (where DMT is made). Forty-nine days is also when the fetus differentiates into male or female gender. Thus the soul's rebirth, the pineal, and the sexual organs all require forty-nine days before they manifest."