The vast majority of living organisms -- even the simplest of bacteria -- have circadian rhythms, an internal sleep clock. The predictable rhythms of night and day help guide human sleep patterns, and when we're deposited into a new time zone, our internal clock isn't always easy to reset, causing jet lag.
But as a newly published study shows, at least one species of fish, Astyanax mexicanus, known as the eyeless Mexican cave fish or blind Mexican tetra, doesn't posses circadian rhythm at all -- its internal clock forever frozen, the first such species found. Teleport the blind fish anywhere in the world, and its sleep won't be disrupted in the slightest.
"Some organisms have stronger circadian rhythms, and some weaker," lead author Damian Moran, a postdoctoral student Sweden's Lund University department of biology, told TIME. "But these fish have none at all."
Moran and his colleagues weren't even intending to study the fish's circadian rhythms, (or lack thereof), they simply wanted to better understand some of the biological benefits of foregoing eyesight. But in the process of exposing both cave-dwelling tetra and a surface-dwelling relative to various amounts of daylight, researchers found blind cave fish always consumed the same amount of oxygen.
In other words, the metabolism of the blind tetra was unaffected by changes in amount of light, whereas its relative's metabolism peaked during daylight hours, even when it was exposed to 24 hours of darkness -- similar to jet lag.
"Not only do we not really know that much about circadian energy use in animals in general, we don't even know how to consider animals that don't have these circadian rhythms," Moran told Live Science. "We tend to assume that these rhythms are always adaptive, that they serve some really important purpose. But what happens in animals that don't have these cycles? It's a real conundrum."
By foregoing an internal clock, and maintaining a steady rate of metabolism, the blind tetra uses 30 percent less energy than the surface tetra.
"We know they save a lot of energy, and that's good if you're living in a cave, because caves tend to be quite food limited," Moran added.
The study was published this week in the journal PLOS ONE.