This is the third column in a series about the importance of adequate sleep for optimal physical and mental health, based on the book “Why We Sleep,” by sleep scientist Matthew Walker, PhD.

Circadian rhythms are repetitive cycles that almost all living organisms go through every 24 hours, driven by an “internal clock” in our brains. Even the trillions of organisms that make up our gut microbiomes have their own circadian rhythms. In humans, circadian rhythms control not only sleeping and wakefulness, but also “timed preferences for eating and drinking, your moods and emotions, the amount of urine you produce, your core body temperature, your metabolic rate, and the release of numerous hormones.”

The biologic clock that drives circadian rhythms, called the suprachiasmatic nucleus, sits in the center of our brains. It is composed of 20,000 nerve cells, which makes it sound like a big structure, but actually it’s tiny, given that there are 100 billion neurons in adult human brains. Soon after dusk, this structure signals an increase in melatonin, sometimes called the “sleep hormone.”

Not all humans have exactly the same circadian rhythm. Last week’s column discussed different circadian rhythms at different ages — for example teenagers tend not to get sleepy until late in the evening and prefer to sleep later in the morning; seniors get sleepy earlier in the evening and often are up at the crack of dawn. In addition, 40% of adults are “morning larks,” preferring to be “early to bed and early to rise” (“making a man healthy, wealthy, and wise” as Benjamin Franklin put it). Thirty percent of the population are “night owls,” and the remaining 30% are in between.

Unfortunately for night owls, modern society favors morning larks, leaving night owls sleep-deprived, resulting in a greater propensity for depression, anxiety, diabetes, cancer, heart attacks, and strokes. Prior to the advent of electricity, there was little to disturb our natural sleep-wake cycles. There are still a few primitive societies on the planet, who are not affected by modern life. These hunter-gatherer tribespeople “fall asleep two to three hours after sunset, around nine p.m.” and they wake up soon after dawn. Some of these tribes have a biphasic sleep pattern, sleeping seven to eight hours at night and napping for thirty to sixty minutes in the afternoon.

Neglecting these innate biorhythms comes with a price. For example, airplane cabin crews who frequently change several time zones experience more brain shrinkage and impaired short-term memory. Shift workers have higher rates of health problems such as cancer and type 2 diabetes.

Ironically, the medical profession is well-known for ignoring natural circadian rhythms. Up until about two decades ago, going through medical school, internship, and residency was a right-of-passage, involving schedules that were mentally and physically unhealthy. One reason for this is that medicine is a rather traditional, hidebound profession—senior physicians had to suffer while training and so they feel new physician-trainees should too. Furthermore, there’s the belief that future physicians should learn to be able to do whatever needs to be done (such as surgery, delivering babies, making life-and-death decisions) no matter how sleep deprived they are.

Dr. Greg Feinsinger is a retired family physician who started the non-profit Center For Prevention and Treatment of Disease Through Nutrition. For questions or to schedule a free consultation about nutrition or heart attack prevention contact him at gfeinsinger@comcast.net or 970-379-5718.