Circadian rhythm regulates the periods of tiredness and wakefulness during the 24-hour cycle. Derived from the Latin “circa diem” meaning “approximately a day,” the body clock is calibrated by the appearance and disappearance of natural light in a 24-hour period.
The body responds primarily to light and darkness and is found in all living things — plants, animals (including microbes) and human beings. Studies using fruit flies have been key to finding the molecular gears of biological clocks and the cells that control circadian rhythms.
According to the National Institute of General Medical Sciences (NIGMS), biological clocks are an organism’s innate timing device.
Very often the term biological clock is used interchangeably with circadian rhythms. Though related, they are not one and the same. Biological clocks produce circadian rhythms and regulate their timing.
Like a switch, light can turn off and turn on genes that control the molecular structure of biological clocks. Changing the light-dark cycles affects the sleep-wake cycle by speeding up, slowing down, or resetting biological clocks as well as circadian rhythm. Besides determining the sleep-wake cycle, circadian rhythms influences hormone release, body temperature, metabolism and other functions.
The biological clock is generated by about 20,000 neurons that form a structure called the suprachiasmatic nucleus (SCN), which is found in the hypothalamus in the brain. These neurons receive signals from the eyes. Besides, light, exercise, hormones, and medications affect the SCN and the setting of the circadian clock.
Circadian rhythms are synchronized with the earth’s rotation by daily adjustments in the timing of SCN, following the exposure to light which indicates the time of the day.
When it’s dark at night, the eyes send a signal to the hypothalamus that it’s time to feel tired. The brain, in turn, sends a signal to your body to release melatonin hormone, which makes us feel drowsy. The onset of secretion of melatonin is about two hours before natural sleep time and peaks during the middle of the night. That’s why the circadian rhythm tends to coincide with the cycle of daytime and night-time.
Circadian rhythm works best when there is regular sleep habit such as going to bed at night and waking up in the morning around the same time on all days, weekends included. Jet lag, daylight savings time, or staying awake can disrupt the circadian rhythm. Researchers are studying how shift work as well as exposure to light from mobile devices during the night may alter circadian rhythms and sleep-wake cycles.
Irregular rhythms have been linked to various chronic health conditions, such as sleep disorders, obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.
People with delayed sleep phase disorder are not able to fall asleep at a normal time at night. They may stay awake until 2 a.m. or later. On the other hand, older people often tend to suffer from advanced sleep phase disorder. In this, old people tend to get very sleepy in the early afternoon and go to bed earlier than normal. As a result, they wake up too early in the morning and are unable to go back to sleep. The third category is people with an irregular sleep-wake rhythm. No matter how they try and how hard they work on it, people with irregular sleep-wake rhythm are unable to set a sleep pattern.
