By Linda Conlin, Pro to Pro Managing Editor
Our daily and nightly biological rhythms are entrained to beneficial blue light (roughly 460 to 490 nm). Exposure to this light in the morning raises our dopamine levels and suppresses melatonin (our sleep hormone). As evening approaches and the light fades in our environment, these wavelengths decline in natural light allowing our melatonin levels to rise so that we fall into a deep sleep and stay asleep. I attended a recent continuing education course by Jackie O’Keefe, ABOC, Circadian Rhythms and Blue Light, during which I learned that there’s much more to circadian rhythm than sleeping and waking.
The primary function of circadian rhythm is to coordinate biological processes, so they occur at the correct time to maximize the fitness of an individual. The primary circadian clock in humans is located in the suprachiasmatic nucleus (SCN), a pair of distinct groups of cells located in the hypothalamus. The SCN receives information about illumination through the eyes. The retina contains photoreceptors, which are used for conventional vision. But the retina also contains specialized ganglion cells that are photosensitive, and project directly to the SCN, where they help in the synchronization of this master circadian clock. In addition to sleeping and waking, other physiological changes that occur according to a circadian rhythm include heart rate and many cellular processes including oxidative stress, cell metabolism, immune and inflammatory responses, and regulation of the stem cell environment.
As Jackie pointed out, this coordination of biological processes means that there are optimal times for different activities. Take as an example, someone who wakes at 6:00am and goes to sleep about 10:00pm. Their deepest sleep is at about 2:00am, their lowest body temperature is at 4:30am. Upon waking, the sharpest rise in blood pressure is at 6:45am, and melatonin secretion stops at 7:30am. The highest testosterone secretion is at 9:00am, and high alertness occurs at 10:00am. For the afternoon, the best coordination is at 2:30pm, the fastest reaction time is at 3:30pm, and the greatest cardiovascular efficiency and muscle strength is at 4:00pm. The highest blood pressure is at 6:30pm, and the highest body temperature is at 7:00pm. At 9:00pm, melatonin secretion starts in the pineal gland, and the 24-hour cycle begins again.
Light plays a bigger role in our daily biological rhythms than many of us realize. Its relationship to the circadian rhythm and bodily functioning has profound implications, and even may influence the timing of medical treatments and medications. Once again, the light, eye, and brain connection cannot be overstated. Thank you, Jackie!
