The study was carried out at the Maxwell H Gluck Equine Research Centre, of the University of Kentucky at Lexington.

Jet lag is a phenomenon encountered by long-distance travellers. But it is not just the length of the journey that's important. The significant factor is the rapid crossing of multiple time zones.

So flying from the USA to Britain is more likely to cause jet lag than flying from the UK to South Africa. Jet lag is due to the conflict between the new cycle of light and dark and the body's natural circadian (literally "about a day") rhythm.

Symptoms of jetlag in humans include disturbed sleep patterns, loss of appetite, lack of concentration and lethargy. It is said to take one day to recover from each time zone crossed.

Human athletes have noticed that jetlag can impair performance. So they take time to adapt to conditions in the country where the competition is to be held.

But what about horses? Reduced food and water intake is not uncommon for the first few days following air transport over three or more time zones. It is unclear whether this is due to the development of jet lag or to the adverse effects of the transportation itself.

In the Lexington Study, Dr Barbara Murphy and colleagues looked at how healthy horses responded to an abrupt change in the light/dark cycle.

Before the start of the investigation six two-year old mares had been accustomed to 12-hour light and dark cycles. They were kept in a lightproof barn for the duration of the investigation and fed every six hours.

Concentration of the hormone melatonin, secreted by the pineal gland to signal to the brain that it is time to sleep, as well as body temperature, were monitored throughout the study. These two factors show a regular daily cycle, and provide a simple way of monitoring the circadian rhythm.

After taking baseline measurements of body temperature and melatonin, the scientists brought the light/dark cycle forward by six hours by turning the lights on six hours early. This change in the daylight cycle would have an effect similar to travelling eastwards across six time zones - for example from Houston, Texas, to London.

They found that the melatonin rhythm reset rapidly. On the first day after the change in the light /dark cycle, the cycle of melatonin concentrations had settled into a stable new rhythm. In contrast, the body temperature rhythm had reset by the third day, although the waveform of the body temperature rhythm had not fully adapted for many days.

This resynchronisation is significantly better than in humans or rodents, suggesting that the horse's central pacemaker resets more rapidly.

"Our studies reveal that in contrast to the human, the horse appears to possess a circadian pacemaker that is more amenable to rapid adjustment to a new photo period - suggesting in turn that their performance capacity at a new destination might be less compromised than in human athletes."

In people, as with many animals, a number of body systems have a natural rhythm, controlled by an internal clock. The "suprachiasmatic nucleus", a small group of cells at the base of the brain is the main pacemaker that controls circadian rhythms. It sets a regular rhythm for many body functions including sleeping, eating and drinking, and body temperature.

Light is an important signal that the body uses to synchronise the internal clock with the outside world.

Rapidly crossing time zones confuses the internal clock. The pre-set rhythms are no longer in step with the outside world. The body clock seems to cope better with flying from east to west. It seems better able to adapt to a longer day than a shorter one.

For more details see:
Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light /dark cycle in the horse. BA Murphy, JA Elliot, DR Sessions, MM Vick, EL Kennedy, BP Fitzgerald J Circadian Rhythms (2007) 5, 5