Circadian rhythms are the things that make up the human’s body clock.  The study of these rhythms goes back to the 19th century but what exactly are circadian rhythms and what do they do?

What are Circadian Rhythms?

“Circadian” is a Latin word that means “around a day”.  The term circadian rhythm refers to the changes in both physical and mental characteristics that occur within a day.  In humans, most circadian rhythms are controlled by the body’s “clock”, which is known as the suprachiasmatic nuclei or SCN.  The SCN is a pair of brain structures no larger than the size of a pinhead that is located in the hypothalamus, and contains around 20,000 neurons. Light reaches the SCN via photoreceptors in the retina.

What Circadian Rhythms Do

The SCN creates signals that travel to several different areas of the brain.  One such area is the pineal gland.  This gland reacts to light induced signals by switching off the production of melatonin.  After dark, the body increases the production of melatonin so that people feel drowsy.  The SCN is also involved with the functions that concern the sleep/wake cycle, including body temperature, hormone secretion, urine production, and changes in blood pressure.

It has been discovered that most people’s natural body clocks work on a 25 hour cycle rather than a 24 hour cycle if they are deprived of light and other external time cues.  However, the SCN is reset by sunlight and other bright sources of light, causing biological rhythms to naturally follow the cycle of the sun, which occurs over 24 hours.  Our circadian rhythms can be affected by external time cues such as alarm clocks, outside noise such as garbage trucks, and when meals are eaten.  These time cues are called zeitgebers, which is German for “time givers”.

Other Things that are Controlled by Circadian Rhythms

As well as the sleep/wake cycle, the body’s circadian rhythms also control such things as appetite, energy, mood, and libido.  Essentially speaking, the circadian rhythms control the timing, amount, and quality of the hormones and neurotransmitters that the body produces and secretes.  These hormones and neurotransmitters then determine factors such as how we feel, our sleep patterns, our appetite, our sex drive, and other things that are related to both mood and sleep.  When the circadian rhythms are working correctly, this is called circadian balance.