Circadian Rhythm Definition:
Circadian rhythm can be defined as inherent cyclical and persistent patterns that recur over a period of approximately 24 hours. This word is derived from Latin words ‘circa’ and ‘dies’ which respectively mean ‘around’ and ‘day’. The study of these is called ‘Chronobiology’.
Circadian Rhythms in human body:
The circadian rhythms involve changes in multiple variables like physical, mental and behavioral, hormonal etc. In human body the circadian rhythms are depicted in various biological processes as sleep-wake cycle, digestion, heartbeat, metabolic rate, hormone production, regulation of core body temperature, cell regeneration, feeding etc.
Reasons and regulation of circadian rhythms:
The environmental factors as solar light, darkness, temperature are the main factors for circadian rhythmicity.
These circadian rhythms are regulated by central nervous system which controls them by biological clock called as ‘circadian pacemaker’. This internal biological clock is a part of the brain known as suprachiasmatic nucleus (SCN) which is made of about 20,000 neurons and is located in hypothalamus area above the crossing of optic nerves. Its proximity to optical nerves confers the property of ‘reaction to light’.
Human sleep cycle:
The human sleep cycle is the best example for depicting circadian rhythms.
Sleep is controlled by neurotransmitters largely acetylcholine (ACH). These act on neurons present in brainstem and spinal cord and signals are produced from the SCN. The sleep/wake cycle of humans is regulated by light and temperature mainly. Any changes in these will either shift or disrupt the sleep/wake cycle.
Endogenous melatonin secreted by pineal gland also illustrates close alliance with the internal circadian sleep rhythm. Melatonin increases during darkness and decreases during daytime. Because of this melatonin is capable of inducing sleep when homeostatic drive to sleep is insufficient. It also helps in adjusting the circadian sleep rhythm to new desired timings.
Disturbances of sleep circadian rhythm:
Many health problems are associated with disturbances in sleep circadian rhythm. These disorders could be temporary like
Seasonal effective disorder (SAD): This is a kind of depression triggered by seasonal changes. Seasonal effective disorder is triggered due to changes in seasons like winter where the duration of day is shorter compared to darkness. In such cases the SAD is referred as ‘winter-onset depression’ where the symptoms like tendency to oversleep, drop in energy level, fatigue start in late fall or early summer and continue to early summer. This is the most common type of SAD. About half a million people suffer with this.
The less prevalent one is ‘summer-onset depression’ which starts in summer and continues to early winter. This is identified by insomnia, agitation, decreased appetite.
The common temporary sleep circadian rhythms include jet lag and problems resulting from working late shifts.
Delayed sleep phase syndrome (DSPS):
This is also called as ‘phase lag’. DSPS unlike jet lag, is a persistent condition resulting due to desynchronisation between body’s biological clock and external environment. Patients with this disorder tend to fall asleep late and subsequently face difficulty in waking up early. The patients with this propensity are referred as ‘Night owls’.
This explains why some people prefer to work late in nights than morning times.
Thus circadian rhythms regulate the sleep and wakefulness cycle and any changes in it produce disorders. Therefore to maintain harmony with the environment these circadian rhythms should be balanced with the normal patterns of the body.