The good. Humans greatest physical attribute is perhaps its ability to adapt to its environment. Cold, hot, sea level, mountain living, city, country, much like a coyote, humans will figure it out and thrive.
The bad. We are so good at manipulating our environment to survive, we have altered our natural rhythms of life that we have built into our cell biology. We are finding this to have some serious health consequences.
First, some science. Chronobiology is the study of circadian rhythms. Humans have built in biological clocks (an innate timing device) in every tissue and every organ. Biological clocks produce and control circadian rhythms. There is a master switch in the brain that is composed of 20,000 neurons called the suprachiasmatic nucleus that controls the biological clocks. This resides in the hypothalamus and receives input through the eyes. This is an important distinction as this will show the significance and power of light.
Light has the ability to turn on and off genes that control biological clocks. Because of this powerful impact light has, it becomes very important how much we get and when we get it. To jump start a day, let sunlight hit your eyes. Sunlight has blue light. It creates a power trigger for alertness. Equally important as the sun sets, eliminate or diminish all blue light to allow the body to start producing melatonin (sleepiness hormone). This also coincides with a slight reduction in body temperature. Staring at a computer or cell phone is just triggering the biological clocks that it still daytime. In the absence of light, humans are still wired for around a 24 hours of sleep/wake. Light just influences how we go about regulating other circadian rhythms in the body.
Our natural circadian rhythm is built around a 24 hour cycle(day). There are also other parts of our biology that have circadian rhythms in it. One of the most important is cell metabolism or energy. Every cell has an organelle called mitochondria in it. Mitochondria are the power houses of energy production. They have a fusion/fission pattern. If this is disrupted, energy production is compromised. This pattern is linked with circadian rhythm of the body. How and when energy is produced is in direct relationship to the health of your rhythm. Disrupt your circadian rhythms and disrupt your mitochondrial ability to produce energy.
Performance can even be potentially thought of as having a rhythm. It has been speculated that peak alertness is around 10am. Coordination around 2pm and peak cardiovascular and strength around 5pm. I have heard these numbers thrown around before but couldn't find much on the origins. Peak body low temperature is around 4am and peak body high temperature is around 7pm. Perhaps they were able to coordinate it with this. I for one have never felt strong in the morning when lifting weights.
Other organs also operate with a circadian rhythm. The liver is a prime example. Liver enzymes that help to convert calories to energy are stopped being produced at night. It is producing enzymes to store energy. To take in a large meal before sleeping then disrupts this natural rhythm. Like light, food then becomes a powerful environmental trigger for supporting the natural circadian rhythm. This is becoming known as time restricted feeding. There have been many health benefits shown for creating a smaller window for eating. Instead of 16 hours of eating, trying to get to 10-12 and even 8 hours. For myself, I've been trying to do 10-12 as this allows me to still support biking/lifting and a social life.
Skeletal muscle makes up 40% of all tissues in our bodies. It also contains biological clocks that influence circadian rhythm. The last decade has brought lots of research on studying how exercise influences this rhythm and whether there truly is an optimal time of day for exercise. There have been many studies of mice that show that exercise influences and enhance the health of the biological clocks, but to date no consensus has been shown at what time or what is "best." Most studies have been done with endurance activities, but resistance exercises have also shown to have an impact on the genes and gene expression of the biological clocks.
As humans we have built in rhythms. These rhythms drive almost all aspects of our physiology. Because we are so amazingly adaptable, we can fight against these natural rhythms for years, perhaps decades, before any "wear and tear" or signs of distress show. There is a difference between adequate and optimal though. Science is in agreement, for optimal health, the circadian rhythm must be respected and nourished.
The 3 most powerful tools we control are LIGHT, FOOD and EXERCISE. Establish routines for all three. There will be arguing over the minutia, don't let this paralyze you. Get 7-9 hours. Eat in a certain window of time consistently. Try to exercise everyday around the same time when possible. Following these three principles will give perhaps the best foundation for a healthier lifestyle.