Circadian Rhythm: It Plays a Crucial Role in Health

Health Professional, Medical Reviewer

The light-dark cycle, often called the circadian rhythm, is more than just a change in the environment around us. It has actually shaped the evolution of most living species.

Adaptation to the immediate environment led to behavioral and physiologic consequences. Hunger, core body temperature, growth, and memory are some of the functions critical for survival. All are determined by this internal circadian schedule.

Circadian means “around one day.” It’s built into all living cells. Many human cells contain genes called “clock” and “B-mal” that produce certain proteins called “Cry” and “Per,” and their interactions take place in a cycle that takes a little longer than 24 hours. Their timed interactions allow different parts of the body to take turns in performing their individual functions. The cycling and timing occurs because there is not enough cellular energy in the body for all systems to act at the same time.

Although many human cells have this time-keeping cue, there is one master clock in the body. It is located in the part of the brain called the hypothalamus and it is called the suprachiastmatic nucleus (SCN). This section of the brain sends signals to other parts of the brain telling them to inform us when we are tired and sleepy, and when we should be (and feel) awake.

I have written several articles indicating the negative consequences of not listening to those signals and the many consequences of sleep deprivation. Let’s now concentrate on the pattern of specific disease processes and how they are affected by the sleep-wake cycle.

Cardiovascular disease

The autonomic nervous system is modulated by two processes: the sympathetic system and the parasympathetic system. They usually have opposite influences on body function. With regard to cardiovascular function, the sympathetic system raises blood pressure, heart rate, and respiratory rate. The parasympathetic system does the exact opposite.

During sleep in the non-REM phases, which constitute 80 percent of sleep time, the parasympathetic system is predominantly active. REM phases, which account for the other 20 percent of sleep time, are a mixture of both sympathetic and parasympathetic activity, with spurts of sympathetic surges.

During waking hours early in the morning, there’s a point in the circadian rhythm that instigates a sudden change. It triggers a sudden increase in both heart rate and levels of epinephrine in the blood. Most healthy individuals are able to adapt readily to this change, while others with underlying heart disease struggle a bit with these natural physiologic changes.

The platelets also respond to an increase in epinephrine and other similar substances by increasing their adhesiveness. This increases the possibility of blood clots that may be dangerous in those with pre-existing plaques in the coronary arteries.

This is the most vulnerable moment for cardiac events.  Indeed, the most common period for acute cardiac events is in early morning. In the case of those who have obstructive sleep apnea, the most common time for a cardiac event is during the night.


Bronchial asthma is quite unstable at night especially when untreated. More severe asthma events occur during the night. That’s because an individual’s respiratory rate and the depth of breaths decrease normally during the night, and there is more tendency for bronchial narrowing at night. This process is exaggerated in patients with asthma.

Other factors including aspiration of oral secretions, parasympathetic tone which tends to constrict the bronchial airways, and supine position, have a more profound effect on asthmatics.

Source: Global Initiative for Asthma


Temporal lobe seizures occur referentially at night, whereas other forms of seizures are precipitated by sleep deprivation. Excess fatigue of brain cells due to sleep deprivation lowers the seizure threshold. In this case, amount of sleep and regularity of the sleep-wake cycle are both important.

Alzheimer’s disease

There is a known association between cognitive decline and circadian rhythm disturbances. Sundowning, which is a predominance of robust, aggressive, and frenetic early evening activity, is one such disturbance. One theory suggests that it’s due to decreased melatonin production. The continuing behavior of being awake at night and sleepy during the day will contribute to the cognitive and emotional deterioration.


Chronotherapy is the use of circadian cycles to determine the schedule of administration of medications according to the nature of the disease. It can involve a drug schedule that imitates the natural changes in certain substances in the body, or it can guide administering medication to coincide with the time when it’s most needed. Another version of chronotherapy is to actually manipulate someone’s sleep schedule.

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