Humans—like many other animals, plants, fungi and even bacteria—have an internal biological clock that keeps our bodies on schedule. It helps us to know when to eat, when to sleep and when to wake. It's the reason many of us are feeling a bit off today, just two days after daylight saving time went into effect.
The clock, in humans at least, runs a little longer than 24 hours in most people, which means that it has to be reset each day by cues from sunlight to match up with the 24 hours of the Earth day. High in the Arctic this can be a bit of a problem as there are months of total darkness and never-ending daylight. Without internal lighting and things like (physical) clocks, computers and televisions to tell a person when a new day has begun, experiments have shown that people's internal daily clocks will gradually become more than 25 hours long.
There is a way around that, though, at least for reindeer. Scientists from England and Norway, reporting in Current Biology, have discovered that reindeer (Rangifer tarandus) have lost their biological clocks. Unlike humans, the reindeer have no daily cycle to their melatonin levels, which are important for the sleep-wake cycle. From BBC News:
Professor Loudon said he believed that evolution had "come up with a means of switching off the cellular clockwork" and that the result was "a lack of internal daily timekeeping in these animals". He commented: "Such daily clocks may be positively a hindrance in environments where there is no reliable light dark cycle for much of the year.
This isn't much help, though, to those of us dealing with this weekend's time change.