In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey Hall, Michael Rosbash and Michael Young for their work on the molecular mechanisms of the circadian clock. Two years after Ohsumi's Nobel for autophagy, once again a fundamental cellular mechanism — this time tied to time itself — received the scientific community's highest recognition.
The circadian clock — from the Latin circa dies, "about a day" — is the molecular timekeeping system present in virtually every living cell. Its progressive disruption with age is now recognised as an aging mechanism in its own right, with measurable consequences for sleep quality, metabolism, immunity and longevity.
The molecular architecture of the biological clock
Its core mechanism is a transcription-translation feedback loop involving a small set of key proteins: CLOCK and BMAL1 form a heterodimer that activates transcription of the Period genes (PER1, PER2, PER3) and Cryptochrome genes (CRY1, CRY2). PER and CRY proteins accumulate progressively, form a complex, and re-enter the nucleus where they inhibit CLOCK/BMAL1 activity — thereby repressing their own transcription.
This negative feedback loop generates an oscillation of roughly 24 hours, running autonomously in every cell, even without external cues. The central clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus — a cluster of 20,000 neurons that synchronises the peripheral clocks of every tissue via hormonal (cortisol, melatonin) and autonomic signals.
Why the circadian clock ages
The amplitude of circadian oscillations shrinks with age: daily variations in circadian hormones, body temperature and physiological parameters become flattened — less pronounced, less precise, less synchronised. This is one of the most robust biomarkers of biological aging. Light-to-clock synchronisation also degrades, partly because of reduced sensitivity of the retina's melanopsin cells. Transcriptomic studies further show changes in the expression of BMAL1, PER2 and CRY1, disrupting the precision of molecular oscillation across many tissues.
Finally, the mitochondrial clock is compromised: mitochondria have their own circadian rhythmicity, and age-related mitochondrial dysfunction disrupts that rhythm, creating a desynchronisation between energy metabolism and cellular time signals.
NAD+, sirtuins and the circadian clock: a fundamental triangle
BMAL1 is a direct regulator of NAD+ biosynthesis: it controls the rhythmic expression of NAMPT, the rate-limiting enzyme in the NAD+ biosynthesis pathway. Intracellular NAD+ levels therefore oscillate on a circadian basis — peaking during active hours and dipping during sleep.
SIRT1 is both regulated by NAD+ and a regulator of the clock: it deacetylates BMAL1 and PER2, directly modulating the period and amplitude of circadian oscillations. SIRT3, for its part, regulates mitochondrial metabolism on a circadian basis — its oscillating activity governs the rhythm of oxidative phosphorylation and ATP production across the 24-hour cycle.
Time and energy are co-regulated at the molecular level. The decline in NAD+ with age doesn't just degrade energy production — it also disrupts the precision of the biological clock.
Consequences of circadian disruption on aging
Sleep deteriorates with age, with reduced deep slow-wave sleep phases. It is during sleep that the brain's glymphatic system clears metabolic waste (including amyloid-β), memory consolidation takes place, and DNA repair runs at its best.
Glucose metabolism also becomes dysregulated: the circadian clock controls insulin sensitivity, glucagon secretion and hepatic glucose metabolism, and its disruption contributes to the progressive deterioration of glycaemic control with age. Finally, circadian immunity loses precision: cytokine production, NK-cell activity and vaccine response vary by time of day, and circadian disruption contributes to immunosenescence and chronic inflammaging.
Chronobiology and nutrition: timing matters
Several studies have shown that the same foods eaten in the morning versus the evening produce different glycaemic, lipid and hormonal responses — partly because digestive enzymes and hepatic metabolic enzymes are subject to circadian regulation. NAMPT's circadian signalling — which governs NAD+ levels — suggests that the timing of NAD+ precursor intake could influence its efficacy. This temporal dimension is precisely what justifies scientific thinking about the formulation and delivery timing of longevity actives.
In conclusion
The circadian clock is a central coordinating mechanism for nearly all cellular processes — from energy metabolism to DNA repair, from immune response to epigenetic control. Its intimate coupling with NAD+ and the sirtuins makes it a relevant biological target in any thinking about precision cellular nutrition aimed at longevity.
The biology of aging has learned something fundamental: time isn't just what passes while we age. It is an active, molecularly encoded parameter that governs the efficiency of every cellular process.
This article is published for informational and educational purposes. It does not constitute medical advice and does not replace consultation with a healthcare professional.
