Jenifer Theresal J1*, Keerthi G2

1Senior Dietician, Kauvery Hospital, Tennur, Trichy, India

2Dietician, Kauvery Hospital, Tennur, Trichy, India

*Correspondence: Tel.: +91

Biological or circadian clock


Circadian rhythms describe biological phenomena that oscillate within a 24-hour cycle. These rhythms include blood pressure, body temperature, hormone levels, the number of immune cells in blood, and the sleep-wake cycle. In this article, we will discuss about the biological clock, its importance and a proper way to follow the biological clock or circadian rhythm to lead a healthy lifestyle.


The circadian or circadian rhythm, also known as the “biological clock”, regulates in every living organism some very necessary and important biological functions, such as the sleep-awakening cycle, hormone secretion, blood pressure and even metabolism!

Biological or circadian rhythm is the daily cycle that drives the lives of every living organism and is characterized by a rhythmic alternation in levels of chemical components and their homeostasis in the body and adaptation of biological functions.

Human Metabolism

Early in the morning

  • Melatonin production stops
  • Increasing blood pressure
  • The intestinal motion is stimulated
  • Increases testosterone


  • There is a faster reaction time
  • Better cardiovascular performance
  • More muscle strength


  • Increase in blood pressure
  • Increase body temperature
  • Suppression of bowel movements
  • Melatonin secretion that regulates the sleep cycle causing somnolence

What are circadian rhythms?

Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle. These natural processes respond primarily to light and dark and affect most living things, including animals, plants, and microbes. Chronobiology is the study of circadian rhythms. One example of a light-related circadian rhythm is sleeping at night and being awake during the day.

What are biological clocks?

Biological clocks are organisms’ natural timing devices, regulating the cycle of circadian rhythms. They’re composed of specific molecules (proteins) that interact with cells throughout the body. Nearly every tissue and organ contains biological clocks. Researchers have identified similar genes in people, fruit flies, mice, plants, fungi, and several other organisms that make the clocks’ molecular components.

What is the master clock?

A master clock in the brain coordinates all the biological clocks in a living thing, keeping the clocks in sync. In vertebrate animals, including humans, the master clock is a group of about 20,000 nerve cells (neurons) that form a structure called the superchiasmatic nucleus, or SCN. The SCN is in a part of the brain called the hypothalamus and receives direct input from the eyes.

Biological-or-circadian-clock-1Fig 1: Circadian rhythm cycle of a typical teenager. Credit:NIGMS

Biological-or-circadian-clock-2Fig 2: The master clock coordinates biological clocks from received light. Credit: NIGMS

How do circadian rhythms affect health?

Circadian rhythms can influence important functions in our bodies, such as:

  • Hormone release
  • Eating habits and digestion
  • Body temperature

However, most people notice the effect of circadian rhythms on their sleep patterns. The SCN controls the production of melatonin, a hormone that makes you sleepy. It receives information about incoming light from the optic nerves, which relay information from the eyes to the brain. When there is less light-for example, at night-the SCN tells the brain to make more melatonin so you get drowsy.

Mid day Meal

Biological-or-circadian-clock-3Fig 2: The master clock coordinates biological clocks from received light. Credit: NIGMS

Dietary Habits  Implications in Metabolic Syndrome while skipping the meals
Eating more frequent meals Increased rate of obesity in adults

Lower LDL in adults consuming >six meals

Inversely associated with poor glycemic control and high cholesterol

Eating in the morning Reduced prevalence of metabolic syndrome
One meal per day Weight loss but no benefit to lipid profile

Increase in fasting serum blood glucose

Skipping breakfast Excess body weight and insulin resistance

Correlated with obesity in adults and children

Correlated with mental health problems in adolescents

Increased odds of metabolic syndrome

Low intake of recommended nutrient intake

Eating irregular meals Increase in risk of CVD in men

Increased prevalence of metabolic syndrome

Increased odds of metabolic syndrome in adulthood

Dawn-to-Sunset Fasting (Ramadan) Decrease in waist circumference

Experience weight loss while caloric intake is the same as a non-fasting day Improved lipid profile in healthy adults

Intermittent fasting Decline in weight

While there is consensus across cultures on what a meal is, i.e., a certain amount of food eaten at a specific time, different cultures across the world follow different meal schedules. In some cultures, snacking at night is more predominant, while others snack more during the day. Moreover, weather can be one of the determinants of a person’s meal schedule. Individuals living in hot, humid weather climates may experience more nighttime eating because they tend to leave their house at night to visit friends and family. Thus, meal timing is an important determinant of their health.

Meal Frequency

Meal frequency is another habit that has gained attention in recent years. There seems to be an association between the number of meals consumed every day and chronic diseases. The frequency of meals seems to affect blood glucose levels, as higher fasting glucose levels are one of the risk factors of metabolic syndrome.

Skipping Meals

Eating meals irregularly is a common practice given the fast pace of society and irregular work schedules. A recent survey of Indians in the workplace reported that approximately 60% of millennials, born between 1988 and 2000, skip lunch to get ahead of work.


In modern societies, circadian rhythm and sleep disruption are perhaps more pervasive than ever. There is increasing evidence of detrimental effects on metabolic function and dietary choices, emphasizing the importance of bolstering circadian system function and addressing sleep disruption. Because an appreciation of the importance of circadian system entrainment and sleep may significantly enhance health and productivity for many individuals, educating key personnel has great potential to benefit society.


Ms. Jenifer Theresal

Senior Dietician