The Circadian Rhythm

How Does It Work?

For those of us of the mammalian persuasion, the circadian rhythm is controlled by something called a suprachiasmatic nucleus (call it the SCN to save time and oxygen). When you think of your body’s internal or biological clock, this is it.

It’s kind of weird to think that we could literally have a clock planted somewhere inside of us, constantly ticking away, so that the body always “knows” exactly what time it is. But this is essentially what the SCN is doing.

Not much bigger in size than the head of a pin and consisting of about 20,000 cells, it’s located deep inside the brain, along the midline, sitting just in front of the hypothalamus and above the optic nerve. The SCN’s role as the body’s onboard clock is so crucial to our temporal orientation, that if it were ever damaged or removed, the circadian rhythm would disappear entirely.

As a timekeeping device, the SCN functions with a considerable level of accuracy and longevity. Research suggests that even if you spent your entire life completely isolated from any external time cues (certainly a depressing prospect), your SCN would continue to tick away on its own, maintaining good approximate 24-hour cycles, virtually through your whole lifespan.

But like most clocks, the SCN falls short of keeping perfect time — running slow by about 10 minutes a day, as we saw before. This is why it’s so important for the body’s internal clock to be continuously reset by environmental cues. Otherwise our circadian rhythm would gradually drift out of phase with our surroundings. And since these rhythms are trying to track the Earth’s day-night patterns, seasonal changes in the length of daytime and nighttime must be compensated for. This synchronizing process is called entrainment and it’s brought about using various external cues called zeitgebers (literally “time-givers” in German). Light is by far the most influential of these, but they also include things like temperature, eating and drinking patterns, as well as patterns of physical activity.

It’s important to properly conceptualize just how these time cues influence the circadian rhythm: the body is not reacting to them passively in the Pavlovian sense. Rather, they are being used by the body’s internal clock as a kind of check or reference point.

To keep itself in sync with the world around us, the SCN tries to interpret the day-night cycle by continuously judging how bright it is outside. It actually has its own direct link to the eyeballs via the retinohypothalamic tract (RHT) which carries signals coming in from photosensitive ganglion cells inside the retina. These cells are different from the familiar rods and cones used for vision. The ganglion cells are there to measure the absolute level of light coming in through the eyes and relay the information on to the SCN.

A Look Inside the Biological Clock

So what makes the suprachiasmatic nucleus tick? How does this tiny clump of nerve cells actually keep time? The same way any modern manmade clock does: by utilizing oscillations. Clocks work by counting the cycles of some regular, repeating action with a constant frequency. Think of the rhythmically swinging pendulum of an old grandfather clock. In the suprachiasmatic nucleus, the oscillation used is something called a gene expression cycle, which takes place inside each individual cell of the SCN.

Here’s a highly simplified explanation of how it works: Genes build and maintain cells by producing proteins, in a process known as gene expression. In 1994, researchers discovered a specific “clock” gene that produces a special protein inside the SCN cells. When this protein builds up to a certain level, it enters the cell’s nucleus and actually switches off the gene that produces it. With production turned off, the clock protein wanes away over a certain period of time causing the gene to reactivate and start up production again — until the protein once again manages to switch off the gene. This cycle of switching on and switching off repeats over and over again inside the cell in a kind of endless feedback loop. What this provides is a nice, regular oscillation, perfect for a working biological clock.

The neurons that make up the SCN fire electrochemical nerve impulses inside the brain called action potentials. They vary in frequency over a 24-hour period, peaking around the middle of the day and decreasing to a low at night. These action potentials work like a kind of radio beacon, broadcasting a signal that travels throughout the brain to many different receivers. One of these is the pineal gland, a pea-sized, pine cone-shaped endocrine gland located nearby in the epithalamus of the brain.

The job of the pineal gland is to release the hormone melatonin into the bloodstream, in response to the signal from the SCN. The relative level of melatonin in the body throughout the day is a primary regulator of our circadian rhythm. Its production peaks at night and stops altogether in the daytime. This is true for all mammals, by the way, whether nocturnal or diurnal (active during the day): in diurnal animals like us, melatonin acts as a signal to go to sleep; in nocturnal animals, it is a signal to become active.

Spread throughout all parts of the body is an extensive system of peripheral clocks or oscillators, which work in a similar way to the SCN cells. They are found in everything from the brain, heart, and lungs, to the liver, kidneys, reproductive organs, and even the skin. They seem to be in constant communication with one another to form a kind of integrated timekeeping network, with the SCN acting as the master clock.

These various “slave clocks” exhibit many different modes of functioning as far as how they set themselves for accuracy: some may be purely free-running while others may be more reactive to specific time cues. There is, for example, some evidence that oscillators in the liver might set themselves by our eating patterns, while oscillators in the skin might react directly to the presence of light on skin.

This is a very elegant system indeed: The suprachiasmatic nucleus functioning as the body’s master clock, taking input signals from the eyes to synchronize itself with the outside environment, and sending out neurological and hormonal output signals over multiple paths to coordinate a network of peripheral clocks throughout the body. The result is a system in which every cell of the body sings in harmony with the eternal cycles of the planet Earth.