Our body clock distorts time to stay on schedule in the heat

You might sweat through a July afternoon, yet still feel drowsy right on schedule when evening falls. A new study shows that our internal clock stays on its 24-hour beat in the heat by subtly reshaping the daily rise and fall of gene activity.

The research was led by theoretical biophysicist Gen Kurosawa at the RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS) in Japan.

Why temperature speeds reactions

Most chemical reactions run faster as the thermometer climbs. This is because heat lets molecules collide more often.

Left unchecked, the basic physics would cause every cellular process to speed up. Yet organisms show temperature compensation, keeping their circadian rhythm nearly the same.

Biologists use something called a Q10 value to measure how temperature affects reactions. Even a small 10°F increase can double the speed of many biological processes, showing just how unlikely it is for a body clock to stay completely stable.

This mystery has intrigued scientists who study biological clocks for decades and drives ongoing research into what keeps them so precise.

Keeping the body clock steady

The classic explanation comes from scientist Colin Pittendrigh. In 1954, he noticed that fruit flies kept emerging right on schedule, even when he changed the temperature in their chambers.

Pittendrigh’s observation convinced researchers that the body clock carries built-in brakes to counteract the thermal push.

Subsequent models invoked competing chemical loops that lengthen or shorten the cycle so their temperature responses cancel out.

These ideas work in theory yet do not fully capture the fine details of real gene-expression curves recorded in animals and plants.

The body clock changes shape

Kurosawa’s team found that the key isn’t adjusting the speed of the cycle. Instead, they discovered that changing the shape of the rhythm – something they call waveform distortion – makes the difference.

At higher temperatures, messenger RNA (mRNA) levels climb sharply, linger, then glide down more slowly. This stretches one side of the wave and squeezes the other.

The total area under the curve – an indirect measure of how much protein is made – stays the same, which helps keep the overall cycle length steady.

Viewed on a graph, the once-smooth sine wave becomes a lopsided sawtooth. That geometry alone delivers the needed timekeeping fix.

A math trick explains time in heat

To explain how this imbalance keeps the cycle steady, the researchers used a math technique called the renormalization group method. This technique is usually applied to complex, chaotic systems like turbulence.

The method teases out the slow backbone of a complicated set of equations, showing that only the downturn of the wave needs to relax to counteract universal speed-ups.

Because the analysis is analytical rather than purely simulated, it offers clear testable predictions about which part of the waveform should stretch when the thermostat rises.

The predictions line up with time-lapse luciferase recordings of clock genes in cultured cells, lending extra weight to the math.

Animal clocks match the model

Fruit flies, mice, and even bread mold fungi show the predicted skew when they grow in warmer rooms, matching the model line for line.

Heat-pulse experiments also shift PER and TIM proteins in the direction the theory foresees, confirming that proteins follow the mRNA wave.

In flies carrying the perL mutation, the distortion is exaggerated and the period actually lengthens, highlighting how a single genetic tweak can upset the balance. That mutant teaches researchers where the tipping points hide in the molecular wiring.

Wave shape affects sleep and shiftwork

A sturdier but less responsive clock sounds good until you cross several time zones or work overnight.

Modeling work suggests that as the waveform grows more distorted, the range of light signals that can reset the clock narrows, making jet lag and shift-work fatigue harder to shake. 

“The degree of waveform distortion in clock genes could be a biomarker that helps us better understand sleep disorders,” Kurosawa said.

Ignoring shape may explain why some experimental drugs that tweak reaction rates never made it to the clinic.

Heat and light warp rhythms

Changes in the shape of the body clock’s rhythm also affect how it responds to winter darkness and bright summer light – suggesting a possible molecular link between temperature changes and seasonal mood shifts.

Insects enduring broad temperature ranges show sluggish adjustments because a heavily distorted wave resists dawn cues – a possibility raised by field studies on temperature-trained flies. 

A distorted wave in January may tell migratory birds when to head south or signal bees to forage later in the day. That knowledge could guide design of smart lighting systems that change spectrum and intensity as temperatures shift.

From shape to treatment options

Researchers now need to pin down the enzymes that slow the evening slide of clock mRNA and hunt for drugs that can nudge that process.

The experts also want to learn whether age, diet, or genetic variation affects how easily the waveform bends. That could open paths to more tailored chronotherapy.

Answering these questions will also clarify whether waveform signatures can serve as early warning markers for metabolic disease.

The study is published in the journal PLOS Computational Biology.

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