Cellular Switch Explains Why Humans Aren't Nocturnal

Study reveals how evolution flipped the timing of cellular activity, enabling mammals to transition from nocturnal to diurnal behavior.

Published on Mar. 7, 2026

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A new study has uncovered a cellular switch that explains how mammals, including humans, transitioned from being nocturnal to diurnal after the extinction of dinosaurs. The researchers found that differences in the activity of the mTOR and WNK kinase pathways, which regulate key cellular functions, cause cells to respond oppositely to environmental signals like temperature and osmolarity, essentially flipping the 'day/night switch' at a molecular level.

Why it matters

This discovery helps explain one of the most important evolutionary events in mammalian history and provides insights into circadian medicine, a growing field that examines how the timing of treatments influences their effectiveness. The findings also suggest that as climate change disrupts environmental factors like temperature, mammals may change the time of day they are active, which could disrupt fragile ecosystem balances.

The details

The study, led by researchers from the MRC Laboratory for Molecular Biology, looked at how cells from nocturnal and diurnal mammals respond to environmental signals. They found that changes in temperature or osmolarity caused the cells to respond in opposite ways, including in fundamental cellular functions like protein synthesis. This divergence was traced to the mTOR and WNK kinase pathways, central signaling networks in cells. When the researchers reduced mTOR function in mice, the animals began behaving more like diurnal species, shifting their active hours to the daytime.

  • The study was published in the journal Science on March 7, 2026.

The players

MRC Laboratory for Molecular Biology

The research institution where the study was led.

Matthew Christmas

A co-author of the study from Uppsala University who found that genes regulating mTOR and WNK have evolved faster in diurnal mammals.

Dr. Nina Rzechorzek

A co-author of the study from Cambridge's Department of Engineering who commented on the broad differences in cellular 'clock' responses between nocturnal and diurnal mammals.

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What they’re saying

“That this radically different response of cellular 'clocks' to the same temperature shift seemed to be broadly true across mammalian species, when comparing those that are more active at night versus those that are more active during the day. We need more research to understand exactly how and why this happens, but it could teach us a lot about how biological clocks work and how they impact health and disease.”

— Dr. Nina Rzechorzek, Co-author, Cambridge's Department of Engineering (Mirage News)

What’s next

The researchers plan to further explore how modifications to the mTOR and WNK pathways can influence circadian rhythms and the potential implications for circadian medicine.

The takeaway

This study provides a key mechanistic explanation for how mammals, including humans, evolutionarily transitioned from being nocturnal to diurnal, with important implications for understanding biological clocks, circadian medicine, and the potential disruptions to ecosystems caused by climate change.