Circadian rhythms are cyclic biological changes involving the physical, mental, and behavioral functioning of an organism, following a cycle of approximately 24-25 hours. Their main regulator is light and darkness in the environment. They are surprisingly ubiquitous, being present in humans, animals, plants, insects, and even microbes. Their study comprises the field of chronobiology.
Circadian rhythms owe their regularity and occurrence to the intrinsic biological clocks operating within many organs of the body, including the brain. These clocks are groups of sensitive molecular groupings within specialized cells throughout the body, and all the clocks are regulated by one master clock within the hypothalamic region of the brain.
Within the hypothalamic region, this master clock is located in the suprachiasmatic nucleus (SCN), containing approximately 20 000 neurons. It receives light-induced impulses from the intrinsic photosensitive retinal ganglion cells (ipRGCs) which are special photoreceptors not capable of forming an image but are able to perceive the presence of light.
Circadian rhythms and individual variation
Circadian rhythms vary between individuals because of the inherent genetic variation between organisms. Each individual is also influenced by cues from the environment, the main signal or zeitgeber being light. Such cues switch specific genes on or off to reset the biological clocks.
Circadian rhythms and physiological functions
Circadian rhythms have significant effects on the timing and duration of several physiological functions including alertness, vital functions such as the heartbeat and blood pressure, body temperature, hormone secretion patterns, and sleep. Their dysregulation can cause insomnia and other sleep disorders. Disruption of these rhythms has also been linked to several metabolic disorders such as diabetes and obesity, as well as psychiatric illnesses including depression, bipolar disorder, and seasonal affective depression.
Artificial light at night, especially when inappropriately timed in relation to the biological clock, can lead to severe disturbances of the biological rhythms. This inevitably impairs normal physiology, which is geared towards preparing for biologically dictated changes in the physical environment and in activity (such as hiding from predators during the night, and working by day).
This internal timekeeping activity also ensures that bodily processes occur in synchrony with the calls on them at various times, such as being alert when activity is expected and winding down when sleep is required. Dys-synchrony between the external and internal environment is associated with immediate or long-term adverse effects on the organism’s health and well-being.
Sleep length, timing, and circadian rhythms
Sleep has been particularly well studied in relation to the circadian rhythms. The SCN secretes melatonin in a specific pattern in response to environmental light. The night causes melatonin production to rise, which triggers the onset of sleepiness.
The sleep-wake schedule differs between individuals, and is most obvious in relation to morning or evening alertness. Some people are morning type individuals (or ‘larks’), and wake early, peak early, but wind down relatively early in the night. Others (the ‘night owls’) wake only by late morning or mid-day, but have peak productivity in the evening or at night and sleep late. This tendency is manifested as early as the first years of school.
The core temperature drop that precedes sleep is different in these two types, as is the rhythm of melatonin secretion. This suggests that the biological rhythms are also fundamentally different in phase between the types, though social needs may impose a superficial similarity on their behavior with regard to sleep timing.
In other words, morning type people may actually go to sleep after the time suggested by their circadian temperature drop, to conform to social convention. It is significant that later research showed a greater difference between these chronotypes with respect to peak and trough alertness, varying from four to six hours.
It has been suggested that a circadian rhythm of less than 24 hours predisposes one to the morning chronotype while a longer-than-24-hours clock may make for the ‘night owl’ chronotype. In addition to genetics, the daily routine, age, and other health conditions, may all contribute to the chronotype.