Biological Clocks – Circadian rhythms, Tidal rhythms, Lunar rhythms, Jet lag, Entrainment

What is Biological Clocks?

Biological clocks are the internal timing system present in living organisms, and it is the process by which rhythmic changes occur in a regular pattern. It is an endogenous mechanism that runs for about 24 hours, and this rhythmic behaviour is referred to as circadian rhythm.

The rhythm continues even in the absence of external cues, but light is the major factor that helps in resetting the cycle. In mammals, the master control is present in the Suprachiasmatic Nucleus (SCN) of the brain which sends signals to different peripheral tissues. It is the system that regulates the sleep–wake cycle, hormone secretion, digestion, and body temperature.

The molecular basis of this clock is maintained by certain genes and their protein products which inhibit or activate each other in a cyclic manner. Biological clocks are found in plants, animals, fungi, and bacteria, and this mechanism helps the organisms to adjust with daily environmental changes.

The understanding of biological clocks has developed slowly, and it is the process where rhythmic behaviour in organisms was observed long before the molecular basis was known. One of the earliest observations was made in 1729 when periodic leaf movements in Mimosa plant continued even in darkness, and this showed that an internal timing system is present. It is during the mid-20th century the study of these rhythms became a defined field, and this is referred to as chronobiology. The term circadian rhythm (circa–about, diem–day) was introduced to describe the endogenous 24-hour cycle seen in living organisms.

In this period, controlled experiments were done in isolated conditions, and it is in these studies that it was shown that humans also maintain daily rhythms even without external cues. These experiments helped in identifying light as an important Zeitgeber (time giver) for resetting the internal clock.

Later, around the 1970s, genetic studies on Drosophila melanogaster identified clock genes such as the period (Per) gene, and mutations in these genes changed the rhythmic behaviour of the organisms. It is the discovery of these genes that proved biological clocks are regulated by a transcription–translation feedback mechanism in which genes and proteins affect each other in a cyclic pattern.

Further studies showed that the Suprachiasmatic Nucleus (SCN) is the main pacemaker in mammals, and other tissues also contain their own peripheral clocks. It was also found that different groups of organisms such as plants, fungi, and bacteria have separate evolutionary origins of clock mechanisms. In cyanobacteria, the clock is maintained mainly by a protein phosphorylation cycle, showing that the mechanisms are not identical in all organisms.

Over the past few decades, extensive research on molecular details of these clocks has established the complete pathway of circadian regulation, and this led to the award of the Nobel Prize in Physiology or Medicine in 2017 to scientists who identified the major clock genes and their interactions.

Types of biological clocks

1. Solar–controlled rhythms– These rhythms are adjusted mainly by sunlight, and it is the process where organisms follow the 24-hour cycle of Earth.
   • Circadian rhythms – It is about 24 hours, and it controls sleep–wake cycle, hormone secretion, body temperature and daily metabolic activities. The SCN in the hypothalamus act as the master pacemaker which keeps other clocks in phase.
   • Diurnal rhythms – These are directly linked with day and night, and organisms show activity or rest depending on light availability.
   • Ultradian rhythms – These are shorter than 24 hours. It is the process where repeated cycles occur many times in a day, such as hormone pulses or stages of sleep.
   • Infradian rhythms – These rhythms last longer than one day. The menstrual cycle (~28 days) is an important example.
   • Circannual rhythms – These run for about one year. It is observed in many animals during migration, hibernation and reproductive periods.
2. Lunar and tidal–controlled rhythms– These rhythms occur mainly in marine organisms, and it is the process regulated by moonlight or the gravitational effect of the moon.
   • Tidal (circatidal) rhythms – It is about 12.4 hours. Marine organisms show movement according to rise and fall of the tide, helping them avoid drying or drowning.
   • Circasemilunar rhythms – These run for about 14.8 days, and it matches spring and neap tides. Many marine organisms release gametes during this period for successful fertilisation.
   • Circalunar rhythms – It is about 29.5 days. Certain organisms adjust maturation or reproductive cycles according to moonlight pattern.
3. Computational or ageing clocks– This is another type where ageing pattern is studied using biological markers, and these clocks are used to calculate biological age.
   • Epigenetic (DNAm) clocks – Based on DNA methylation pattern at CpG sites, and it indicates biological age more accurately than chronological age.
   • Proteomic clocks – These depend on the concentration of different blood proteins that change with ageing.
   • Microbiome clocks – It is based on gut microbial diversity, and loss of certain bacteria indicates faster ageing.
   • Inflammatory or immune clocks – These clocks measure inflammation load and show how ageing affect immune functions.
   • Neuroimaging or echo clocks – Brain or cardiac imaging data is used to estimate internal ageing rate of organs.

Advantages of biological clocks

• It is helping in maintaining the daily timing of different activities in the body.
• It is the process that allow organisms to adjust with the light–dark cycle.
• It is useful for keeping the sleep–wake cycle in proper order.
• These are important for synchronizing hormone secretion with day and night.
• It is controlling body temperature changes during the 24-hour period.
• The major source of metabolic regulation is controlled by the biological clock.
• It is helping in maintaining feeding and activity rhythm.
• It is the process that coordinate different cellular functions so that incompatible reactions is separated in time.
• Among the important advantages is protection of cells by timing DNA repair when damage is less.
• It is helping in maintaining growth and developmental timing in organisms.
• These clocks is supporting immune activity by keeping immune cells more active at specific times.
• It is useful in reducing disease risks because proper timing of physiological processes is maintained.
• Some of the main advantages include better response to medicines when treatment is given according to body timing.
• It is helping in predicting the best time for drug action in chronotherapy.
• This is referred to as an internal timing system that maintain overall stability of the organism.

Uses of Biological Clocks

• It is used for maintaining the sleep–wake timing in organisms.
• It is used to regulate daily metabolic activities and energy use.
• It is helping in controlling hormone release at specific times of the day.
• It is used for timing of body temperature changes during the 24-hour cycle.
• It is helping in coordinating feeding behaviour with the active period.
• It is used in maintaining proper timing of cell division and DNA repair.
• These clocks is used for adjusting behavioural patterns with environmental light conditions.
• It is used in chronotherapy for deciding the correct time of giving medicines.
• It is helpful in improving immune responses at appropriate times of the day.
• It is used for supporting growth and developmental timing in organisms.

FAQ