The discipline that studies the relationship between time and the biological rhythms of people is called chronobiology. The specialists consulted point out that knowing the biological schedule of an individual allows, just to mention a few examples, that the patient responds better to a medication, to exercise or to the absorption of a certain nutrient.
Although each person has their own habits, and they must be studied individually (personalized medicine), in all cases the passing of the hours of the day (24-hour circadian rhythm), generates changes in the various mechanisms that the body has to regulate its operation.
Fernando Botto, chief cardiologist of the research area of the ICBA Cardiovascular Institute, explains that circadian rhythms compromise all the mechanisms that the body has organized to survive in its healthy functioning.
The best time to study, exercise and have sex
“When we sleep it is a stage in which glucose, triglycerides and nutrients accumulate in the tissues. And when we wake up, the body starts up and, among other things, increases the pressure and heart rate. Circadian rhythms are governed by the brain with the autonomic nervous system, which is what opens the pupils, generates saliva. That is, we do not control it,” says Botto.
The study of chronobiology, according to Daniel Cardinali, senior researcher emeritus at Conicet and professor emeritus at the University of Buenos Aires, is of vital importance. Knowing the biological schedule of each individual, he says, makes it possible to understand the specific moment when they can best respond to a treatment, such as cancer therapy, and thus determine when the toxicity of a drug can be minimized and its effectiveness increased.
We put together a guide on how circadian rhythms impact the heart, brain, and diet. Although, the specialists emphasize that, although general recommendations can be given, it is convenient that each case be analyzed in a particular way.
What happens in the morning
When a person wakes up, Botto describes, there is usually an increase in blood pressure and heart rate. That’s because the autonomic nervous system releases chemicals like cortisol and adrenaline. It also increases the level of glucose and insulin in the blood.
This activation is physiological and depends on the “internal clocks” that prepare the body to start the day, but for vulnerable people, with poorly controlled risk factors (for example, cholesterol, pressure, weight), it is a propitious scenario to suffer a heart attack or stroke. Therefore, the highest incidence of heart attacks occurs between 6:00 am and 10:00 am.
“When it is said that someone died sleeping, it is possible that they woke up at dawn and that the physical reaction generated by waking up has collaborated in the death,” says Botto.
As for exercise, although it is advisable to do it at any time of the day, Botto points out that it would be optimal to train in the morning. “Aerobic exercise is healthy at all times, but it would seem that in the morning it adapts better to internal rhythms since many metabolic mechanisms are activated to produce energy and thus the cardiovascular benefits would be greater.
In the same way, it would be better to consume the largest number of calories in the morning and not at night, as is customary in our region. Doing sports at night is also healthy, only the body will be left with a high level of adrenaline and then it will be more difficult to fall asleep”.
For pressure control, Botto recommends that patients not take their pressure at the same time of day. The test should be performed when the person feels well, because, he clarifies, it is one thing to be hypertensive and quite another to have a circumstantial event of high blood pressure, motivated by some bodily ailment, worry or stress, which should not be taken as real high blood pressure.
Regarding the time to take antihypertensives, it indicates that some specialists recommend using them in the morning so that the drug has its peak of action at the time of greatest cardiovascular risk. However, he points out that there is a debate on this issue and there is no definite recommendation. Each doctor decides with his patient, in a personalized way.
Pablo López, a member of the Department of Cognitive Psychotherapy at INECO, explains that the cycles of the brain are broader and are divided into sleep and wakefulness. These cycles, he adds, are within the aforementioned circadian rhythms, and that is why it is said that sleep is regulated by a biological clock.
At the brain level, this clock is located in an area called the suprachiasmatic nucleus (SCN) and sends signals to other brain structures associated with sleep regulation, indicating and differentiating the moments in which the predisposition to sleep increases or decreases. To do this, the hypothalamus, the structure where the SCN is located, sends signals to stimulate or inhibit melatonin production.
Presence and absence of light
“At the environmental level, the previously described circuit is regulated by the presence and absence of light. In this sense, the lack of light predisposes us to sleep because, among other things, it increases the production of melatonin. On the other hand, when we wake up in the morning, the opposite process occurs: melatonin decreases and cortisol and glucose, associated with energy and activity, rise, making it easier for us to wake up”, adds López.
Regarding sexual energy, Cardinali describes that lack of sleep, especially in men, decreases libido. The specialist emphasizes that, for them, the morning should be the time of greatest sexual desire.
“The variability of sexual desire in men is linked to testosterone levels, whose peak occurs in the second half of sleep. Therefore, the morning should be the time of greatest sexual desire. In the case of women, libido is linked to another chronobiological aspect, which is not the hours of the day, but is linked to the 28-day menstrual cycle. The desire grows in the periovulatory stage (half of the menstrual cycle), and in the premenstrual moment”.
New ways to address metabolic health
Monica Katz, founder of the Durand Hospital Eating Disorders Team and author of the book “The No Diet Method”, believes that nutrition science could generate new ways of addressing metabolic health and obesity through the implementation of dietary guidelines aimed at meal times.
Eating breakfast, Katz argues, is accepted as beneficial, despite the fact that many people do so-called intermittent fasting. According to the specialist, breakfast can increase energy, concentration, memory and problem-solving skills. It also improves metabolism, digestion, and bone strength and helps lower the risk of heart disease, high blood pressure, and obesity.
“A recent meta-analysis showed an association between energy intake at night and higher average body mass index. Obese people consume most of their calories in the afternoon and evening period, compared to lean people. Calorie intake in the morning (50% calories at breakfast and 14% at dinner) resulted in 11 pounds greater weight loss relative to calorie intake at night.
In addition, the fact that the highest caloric intake is in the morning increases satiety, improves blood glucose, insulin and triglycerides and decreases hunger compared to the consumption of calories at night,” says Katz.
Double energy expenditure on breakfast
Katz cites the “Big Breakfast Study”, the results of that research related to caloric expenditure showed that the body expends more than twice as much energy when digesting breakfast than during dinner.
On the other side of the divide, on whether or not to eat breakfast, there are those who defend intermittent fasting. “In my experience intermittent fasting is safe for some. For others, especially those with a history of eating disorders and some altered body image, it can represent a dangerous path towards unhealthy patterns of restrictive eating, “says Katz.
Body reaction in the evening
In the afternoon, López describes, there is another “milestone” during the sleep-wake cycle that usually occurs around 2:00 p.m. “It is common that at that time we notice that sleep also increases, and this does not have to do only with the fact of having eaten or not, but with the fall of the mechanisms responsible for maintaining alertness.
In this sense, the process of the circadian rhythm coexists with the process involved in digestion, through which information reaches the central nervous system from the digestive tract and through the vagus nerve, which inhibits the alert mechanisms of our body”.
When is there a greater brain predisposition to learn and create?
In human affairs it is difficult to speak of general rules. One of the aspects to take into account when referring to the level of activity or predisposition to do things is that of the chronotypes, which can be defined as the predisposition of each person to have peaks of energy and rest.
“There are people who tend to prefer the morning schedules (morning chronotype or larks) and others the night schedules (late chronotype, commonly called owls). However, the majority do not fall into either of these two extremes, and have an intermediate chronotype (approximately 50% of the population)”, concludes López.
Low heart rate and blood pressure
In the afternoon the fall of the morning action begins. Lowers heart rate and blood pressure. Although in the morning metabolic activity increases to increase energy (catabolism). In the afternoon what is called anabolism begins, which is the process by which fat deposits begin to be generated. None of this implies, Botto warns, that there are no heart attacks in the afternoon, but the percentage is usually lower than in the morning.
Katz points out that it has not been studied that a person should wait a certain number of hours after lunch to eat again. “But it is convenient, in the case of compulsive people with a history of hunger memory, that they eat more frequently. Everything will depend on the profile of the person’s eating behavior,” says the specialist.
Night shifts and weight gain
“Night workers are predisposed to weight gain and metabolic disorders. Although there is no specific time, the data points to 7:00 p.m. as an optimal healthy limit, which is related to the biological clock”, argues Katz.
At night, Botto explains, the pressure should drop. And if this does not happen, it is likely that it will collaborate as a trigger so that the person can suffer a cardiovascular event. “During the night the pressure should drop between 10 and 20%. If it doesn’t, there’s an example of circadian rhythm disturbance. When one requests a pressure measurement or 24-hour ambulatory monitoring, one of the indicators we look at is heart rate variability.
If it is high, it means that the autonomous system works well, but if little variability is observed, it can become dangerous. Regular exercise and weight control help improve this variability, and thus strengthen a protective mechanism. When for some reason circadian rhythms are altered, the risk of a heart attack increases. Because of this, people who work nights can have their circadian rhythms out of balance. And have a greater tendency to diabetes, obesity, cognitive disorders and cardiovascular risk”, indicates the specialist.
Theories of aspirin use
Regarding the use of aspirin, he maintains that there are theories that indicate that at night it would be the most appropriate. This to attack the increased platelet activation and risk of morning clot formation. However, he stresses that there is no clear guideline to follow. What he suggests is that aspirin, where indicated, be taken at any time of the day with meals.
In the case of adults, the recommendation is to sleep between 6 and 8 hours a day. However, there is great variability among people regarding the hours they need to sleep. A central aspect that is complemented by the recommended number of hours is the stability of the schedule. Sleeping 8 hours on a regular schedule is not the same as sleeping the same hours at different times of the day.
“Our body is designed to sleep at night and be awake during the day. However, for different reasons, some people are awake at night and sleep during the day. These people require extra effort to stay awake during activity because it goes against our physiology. Some studies even indicate that those who work at night, even sleeping the recommended hours. They do not recover in the same way as those who sleep at night,” says López.