Science claims ignorance in regards to sleep. Humans simply don’t know what sleep is for.
But I do. The primary function of sleep is cognitive in nature, specifically, regarding the function of memories and memory making. The human brain is finite. It only has a certain capacity to store memories, which can range in vivid, real like simulations, or dull, bland generalities without much reference to reality. Different people have different brains. And different brains have different memory functions. For example, some autistic brains are capable of remembering and recalling vast amounts of specific data. Some autists can remember numbers, and recite pi digits almost endlessly. Other artistic autists can see a building one time, and redraw the whole picture almost to every detail. One woman even demonstrates that she can remember almost every part of every day of her life. Her brain has a “super memory” if you will.
The brain can specialize in the focus of a memory, remembering extraordinary details. Or the brain can specialize in amount of memories, by recalling less specific details but more generalities. Therefore, different brains have different memory functions. This is demonstrated by a crowd of individuals who witness a spectacle, like a theatre play or a movie. Individuals relate with different characters more or less, and different events.
This represents cognitive bias.
However, there are general rules in regard to memory science. Different individuals react to stress differently. When presented with an emergency scenario, some people will run toward the danger (fight reflex), some people will freeze in place (playing dead reflex), and others will run away (flight reflex). This is all based on instinct. When a great calamity like war, terrorist attack, or natural disaster occurs, different people will react according to their genetic instincts. There is almost no cognitive bias in these instances.
And traumatic experiences are very stressful. This imposes a large “force of memorization”. In life and death situations, people tend to switch to survival, instinct mode, and can be traumatized from them. And the traumatization is the primary concern of “psychology”. Different life changing events occur to different people, producing different reactions, and most importantly, different memories.
This returns to sleep science in a specific, simple, common sense way. Sleep is the realm where daily life becomes “integrated” into the subconscious, or to the deeper level of the unconsciousness. This is how reflexes and instincts are comprehended, and possibly changed, over time and across generations. Sleep, stress, and memories are all related, and occur during the dream state. This dream state is one where your essence, your “soul”, integrates memories into the deepest level of your consciousness and your existence.
Memories are kept or forgotten, like a form of material wealth, according to how your genetic, reflexive, instinctive cognitive bias interpret reality. For example, one person feels stress, anxiety, immense and fear in a warzone. He cannot function. While another, experienced soldier feels relaxed, and even may enjoy the experience. This is because one soldier might be a “natural born” soldier, who shines on the battlefield, while the other soldier is a coward and an unfit soldier. Why is one individual better fit for war than another? It is probably genetic.
But it absolutely must represent the cognitive bias inherent with stress and memory making.