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Getting out of bed some days feels like too much effort. If only night lasted all day, just like it does for the blind Mexican cavefish. Like the fish, you just might save some energy by living in an endless night.

The Mexican tetra (Astyanax mexicanus) is a species of fish found in the southern United States and parts of Mexico. It takes two forms, or ‘morphs’ – one with good vision and one that develops without eyes. While the sighted morph swims in streams exposed to sunlight, the blind morph can be found underground in lightless caves, finding its way by detecting changes in the surrounding water pressure.

Like night and day

A world without sunlight is also a world without the predictable rhythm of night and day. Many living things – from humans to flowers, and even a number of microscopic organisms – have chemical processes that roughly match the 24 hour patterns of night and day. These processes help prepare you with a boost of energy during the times of day you need it most.

Called a circadian rhythm, this body clock doesn’t rely on you checking your watch or even seeing the Sun. It is kept in check by periods of light and dark. If you’ve ever had jet lag, you’ve experienced your circadian rhythm telling you to sleep or eat at odd times of the day.

Cavefish clock?

Yet the Mexican cavefish lives in darkness, so does it even have a circadian rhythm? To find out, Swedish biologists compared the blind morph with the sighted morph, and found that the blind morph does not have this internal body clock. It also uses less energy than the sighted morph, by not having to prepare for daylight.

Next time you oversleep, you’ve got a new excuse. Tell your teacher you’ve become a blind Mexican cavefish and lost your body clock!

More information

Why does sunshine make me sleepy?
Check out this cool CSIRO tech that can map caves

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