A blind, cave-dwelling fish in Somalia knows what time it is, but its "day" is twice as long as ours.
Most animals have an internal body clock, or circadian rhythm, that lasts around 24 hours and is modified by the light-dark cycle of a day.
But an international team, whose research is published in the open access journal PloS Biology, shows that certain blind cave fish have a circadian rhythm that lasts almost two days.
The cavefish, Phreatichthys andruzzii, has evolved for nearly two million years in the isolated darkness of caves beneath the Somali desert.
Professor Nick Foulkes, of the Karlsruhe Institute of Technology in Germany, said that this particular species was chosen "because it was such an extreme example, having been isolated from a day-night cycle for so long".
In the course of its evolution it has lost its eyes, colouration and scales, having no need for them in the pitch-black of an underground cave system.
But it appears that the absence of day and night has caused a much more profound change in the fish's life rhythm.
The internal body clock of most mammals is slightly longer than 24 hours, although it is unique for each person and is modified by light.
This is most obvious to us when we travel across time zones, as jet lag is caused by the delay in our circadian rhythm synchronising with the new daylight times.
On a smaller scale, the body clock can be measured by the switching on and off of certain "clock genes" at different times during the day. This happens automatically daily, but is synchronised with the day-night cycle through exposure to light.
Light is detected primarily by the eyes, but most cells in the body have some reaction to light levels. In non-mammalian animals, such as fish, these "peripheral" detectors play a more important role.
This means that, even though the cavefish have lost their eyes over the course of evolution, their bodies should still be able to react to changes in light.
When comparing the reactions of the circadian rhythm of the cavefish to those of a "normal" zebrafish, however, the blind fish showed none of the responses to external light changes that the zebrafish did.
After two million years in the dark, the cavefish have no need to react to the light, and their body clocks have permanently changed to reflect this.
But these blind fish do still have a body clock, which can be reset by triggers other than light.
Feeding the fish at regular times showed that both the zebrafish and the cavefish responded by resetting their circadian rhythms.
Furthermore, when the cavefish were left to reset their clock according to their natural rhythm, the researchers found that their "day" is 47 hours long.
Professor Foulkes said that this was "possibly linked with food availability, or we could have caught them in the process of losing their clocks. If we look again at them in a few million years, they may have no trace of a circadian rhythm".
The team plans to investigate whether this gradual loss of body clock is a common feature among all species of fish living in perpetual darkness.