Light and darkness are primary tools for controlling the brain and nervous system's state, determining wakefulness or sleep.

“The main levers and tools that are going to allow you to control when you are awake and when you are asleep and to get better sleep every single night are light, literally photons, light energy, could be from sunlight, could be from artificial light, we will discuss those particulars in a moment, as well as darkness. That is the absence of light. So we've got light and dark. Those are two very powerful tools to encourage your nervous system to be in one state or another, meaning awake or asleep.”