Concept

The Circadian Clock

Intro

Your body keeps time. Not by watching the sun, though it checks the sun to stay accurate, but by running an actual clock made of molecules inside your cells. This clock ticks with a period of about twenty-four hours, and it governs an enormous amount of your biology: when you feel sleepy and when you feel alert, when your body temperature dips and rises, when hormones are released, when digestion ramps up, even when certain genes are switched on and off through the day. Every cell has a version of this clock, and a master clock in the brain keeps them all in step, like a conductor keeping an orchestra together, while sunlight resets the whole system each day so it does not drift. The clock is built from a small set of genes and proteins wired into a feedback loop that makes them rise and fall on a daily cycle. A molecular timepiece, running in every cell and synchronized across the whole body, is the kind of precise, integrated control system that reflects engineering.

In full

The circadian clock is a molecular oscillator built on a transcription-translation feedback loop. Two proteins, CLOCK and BMAL1, pair up and switch on the genes that make two other proteins, PER and CRY. As PER and CRY accumulate, they pair up in turn and feed back to shut off CLOCK and BMAL1, so their own production falls; then, as PER and CRY are degraded, the block is lifted and the cycle begins again. This rise-and-fall loop takes roughly twenty-four hours per turn, a delay set by the timing of protein production, modification, and breakdown. The loop runs in cells throughout the body, called peripheral clocks, and these are coordinated by a master clock in the brain, the suprachiasmatic nucleus, or SCN, a cluster of neurons that keeps the peripheral clocks in phase and aligned with the outside world. The SCN receives input from light-sensing cells in the retina, so daylight entrains, or resets, the clock each day, correcting the small difference between the clock's natural period and the true twenty-four-hour day and keeping internal time locked to solar time. Through this circuitry the clock schedules sleep and wake, body temperature, hormone secretion such as the timed release of cortisol and melatonin, metabolism, and the daily on-off cycling of a large fraction of the genome. The core mechanism is deeply conserved across life. The 2017 Nobel Prize in Physiology or Medicine recognized the discovery of the molecular loop underlying it. See Irreducible Complexity and Specified Complexity.

The mechanism

• The core loop. CLOCK and BMAL1 join to switch on the PER and CRY genes; the resulting PER and CRY proteins then pair up and switch their own activators back off, producing a self-resetting cycle.
• A built-in delay. The time it takes to make, modify, and degrade the proteins sets the period of the loop to roughly twenty-four hours, so the clock ticks on a daily rhythm without any external timer.
• Peripheral clocks everywhere. Nearly every cell in the body runs its own copy of the loop, timing local processes in tissues from liver to skin.
• The master clock. The suprachiasmatic nucleus in the brain acts as the central pacemaker, keeping the many peripheral clocks synchronized with one another and with the day.
• Entrainment to light. Signals from light-sensing retinal cells reset the master clock each day, correcting the clock's slight drift so internal time stays aligned with the sun.
• Whole-body scheduling. The clock times sleep and wakefulness, temperature, hormone release, metabolism, and the daily cycling of a large share of the genome.

Why this points to design

A clock is a device for keeping time, and keeping time well requires several matched features working together: an oscillator that reliably repeats, a period tuned to the interval you care about, a way to distribute the time signal so everything stays in sync, and a way to correct drift against a reference. The circadian system has all four. It has a molecular oscillator, a feedback loop whose components are timed to complete a cycle in about a day; it has a distribution and synchronization scheme, the master clock keeping the peripheral clocks in phase; and it has an entrainment mechanism, light input resetting the system daily so it does not wander. These are the same functional requirements a human engineer designs into a network of synchronized clocks. Remove the feedback loop and there is no oscillation; remove the master pacemaker and the cellular clocks fall out of step; remove entrainment and internal time drifts away from real time. The parts are jointly required for the function of accurate, body-wide timekeeping, and their integration into a single coherent system that schedules physiology across every tissue is precisely the kind of purpose-fitted engineering that points to a designer. See Irreducible Complexity and Intelligent Design.

The evolutionary account, and why it falls short

The evolutionary account proposes that clocks arose because life faced a predictable daily cycle of light and dark, so any molecular process that happened to oscillate near twenty-four hours conferred an advantage, letting organisms anticipate dawn, protect vulnerable chemistry from daytime damage, and time activity, and natural selection then refined a chance oscillation into the tuned feedback loop, adding the master pacemaker and light entrainment over time.

The account explains why a clock would be useful without explaining how the integrated clock came to exist. Usefulness is not a mechanism; that a reliable twenty-four-hour timer would help does not show how a self-sustaining feedback oscillator, tuned to the right period and wired to a synchronizing pacemaker and a light-resetting input, was assembled. The advantage the story invokes accrues only once the system already keeps accurate time, but each piece is close to useless alone: an oscillation with no way to reset drifts out of alignment and misleads the organism; cellular clocks with no master pacemaker run out of phase and issue conflicting schedules; a pacemaker with no functioning oscillator has nothing to distribute. So the very benefit that selection is supposed to have chased is not available until the matched components are already integrated, which is the point at which there is nothing left for a gradual account to build. Noting that daily rhythms are advantageous, and that clock genes are shared across life, describes the destination and the parts list; it does not demonstrate the selectable, working intermediates or the specific changes that turned a chance oscillation into a tuned, synchronized, self-correcting timekeeping system. That integration is the fact in need of explanation. See Common Descent Critique and Fine-Tuning Argument.

See also

• 50 Amazing Facts About the Human Body, the hub this spoke belongs to
• The Genetic Program of Development, another timed control system in the body
• The Human Heart, a rhythmic system the clock helps schedule
• Irreducible Complexity, why the clock's matched parts are jointly required
• Specified Complexity, functional information as a design signature
• Fine-Tuning Argument, the tuned twenty-four-hour period against the solar day