Want to beat the boss at golf? Schedule your round for 2.30 this afternoon—your co-ordination will be at its sharpest. For an accurate reading of your blood pressure, get it checked at 6.30PM. When was your bowel movement? If it came at 8.30AM, you’re smack-bang in the middle of the scatological bell-curve.
Almost all your physical, mental and behavioural shifts are metered in 24-hour increments by circadian rhythms: internal metronomes sensitive to light, darkness and temperature. They tell your body when to sleep, when to wake, when to release hormones, when to adjust your core temperature and much, much more.
Is that the same as your ‘biological clock’? No, that’s separate, but related: biological clocks are networks of interactive molecules in cells throughout the body, which are themselves governed by a master clock in the hypothalamus of your brain. So, the mechanism looks like this: your master clock co-ordinates your various biological clocks, which in turn set your circadian rhythms.
Your master clock was set billions of years ago. There’s one ticking away inside most organisms on the planet. Major taxonomic groups tend to run their own mechanisms, but in 2012, researchers found one common to mice, fruit flies, a plant, a fungus, an alga, bacteria—even the most ancient things of all, the archaea. Some of these groups evolved circadian regulation about 2.3 billion years ago, soon after the atmosphere began to swamp with dioxygen in the Great Oxidation Event (GOE). Free oxygen is toxic to anaerobic creatures, and the rising concentration probably drove many extinct by dismantling and corrupting atoms crucial to their metabolism. But cyanobacteria were among the first to fight back: studies of species alive at that time have found vestiges of the earliest known pacemakers.
The organisms that survived the GOE did so by evolving the enzyme peroxiredoxin (PRX), which neutralises the poisonous effects of oxygen, and it works as well today as it did two and a half billion years ago.
At least 10 per cent of your genome is regulated by circadian genes.
A flux of ribonucleic acid (RNA) molecules and circadian proteins oscillates predictably over a 24-hour period, switching synapses, hot-wiring hormones, tripping thermostats. Little wonder that when our internal rhythm is lost, or external stimuli change, we feel the effects.
Ordinarily, we take subconscious cues from light and temperature, which in a natural environment march in fairly predictable lockstep. Many animals use them to calculate time, but fruit flies confused by artificial anomalies react in interesting ways: when the cycles were offset by a couple of hours, the flies’ body clocks averaged the differing signals and life went on as usual. But they coped with bigger deviations in temperature by ignoring them, recalibrating their default schedules to daylight cues alone.
Humans, too, struggle when natural orientations change. Non-24-hour sleep-wake disorder plagues blind people. Jet lag is the travellers’ curse, shift-work sleep disorder is a modern workplace malady. Some people even have to keep interplanetary time. A day on Mars lasts 24 hours and 39 minutes, which means the NASA teams controlling Martian rovers end up working otherworldly hours: those extra minutes push their sleep schedules back every day—call it mission creep—so that 1AM goes from dream time to breakfast time over the course of a week. Crews keep Mars time by running blue lights on their desks, which send wakefulness signals to their brains via photosensitive cells in the eye involved in circadian time management.
In 2010, scientists tested the mechanisms of volunteers who worked a 6AM–3PM shift one week, a 3PM–midnight shift the next, then returned to the morning shift on their third week. The study subjects progressively rose later, until their sleep schedules were four hours askew. The researchers plucked head and beard hair from the volunteers throughout, analysing levels of transcribed RNA for peak circadian gene expression, and found that, at the experiment’s end, clock-gene activity was still five hours slow.
Around 25 per cent of the Western workforce clocks in while the rest are asleep, and epidemiologists note they suffer higher rates of breast cancer, metabolic syndrome, obesity, bone dysfunctions, cardiovascular disease, stroke and sleep impairments.
Our body clocks evolved in a time without streetlights or retina screens, and for hundreds of thousands of years, they’ve kept good time. But nowadays, three-quarters of the global nocturnal population is exposed to artificial light, and that could be making our clocks run fast, say scientists. Recent research suggests that disturbances to the circadian clock in our brains may well be accelerating ageing, exposing us prematurely to age-related health risks. In other words, our brains don’t crave variety: deep down, they’re hardwired to keep to the same rhythm, day-in, day-out.