Light, circadian neuroscientist Steven W. Lockley has explained to me, is a fundamental component of our biology. We need the daily 24-hour light/dark cycle to stay properly synchronized with the world around us. Light, Lockley says, is the most important environmental time cue for resetting our circadian clocks each and every day. That’s why the workings of the eye’s nonvisual light-sensing system are so important.
Light travels to the brain’s 24-hour clock, housed in an area of the brain called the suprachiasmatic nucleus. The SCN is made up of about 50,000 cells, each of which is an individual oscillator, or clock. Together, these cells work to control our physiological and behavioral functions that affect, among many things, alertness, performance and reaction times, heart rate, temperature, glucose and insulin levels, and many genes. Lockley explains that the clock naturally runs at a period close to, but not exactly 24 hours (on average about 12 minutes longer, or 24.2 hours), and has to be reset to 24 hours each day by light.
In recent years, researchers have also discovered circadian clocks in the body’s tissues and major organ systems — the heart, the lungs, the liver, the stomach, the ovaries, the pancreas and many more, which, Lockley beautifully details, act as members of the body’s orchestra, keeping time in the peripheral tissue but under the guidance of the conductor in the SCN.
Essentially, the body is one big ticking clock: For the human machine to run as smoothly as possible, we need properly timed exposure to environmental light. In an ideal world, we would be on natural Earth time, resetting slightly differently each day and through the seasons, not on our constant clock time.