By David Rothery
If you've ever flown across the Atlantic, you probably sank into your long-overdue bed bleary-eyed and still woke up far too early the next morning. This is jet lag.
The star athletes of the horse world will be congregating in Kentucky, USA for the World Equestrian Games this weekend, with teams coming from as far as Europe, South America and Australia.
So do horses suffer jet lag when they are flown across the globe to compete?
According to research, the answer is no: horses generally do not arrive feeling grumpy and sleep-deprived like the average human.
Human bodies are used to operating on a 24-hour cycle, and when we move to a different time zone it is said that we need one day per hour of time difference to adjust to our new circumstances.
A horse's sleep pattern is not like ours. An adult horse typically sleeps for only three hours during a 24-hour period, and this is made up of many brief snatches of sleep lasting only a few minutes each.
And although it will sometimes lie flat-out, a horse does most of its sleeping standing up.
David Rothery: Horses recover from jet lag quicker than humans
Easier on four legs than on our two, you might think, but a sleeping horse will usually carry most of its weight on just three.
It stands on two fore legs and one hind leg, with the other hind leg raised and touching the ground only on the point of the hoof.
A horse can maintain this position effortlessly, thanks to an ability to lock its leg joints into place.
If you think about it, it makes sense for a prey animal to sleep on its feet ready to flee predators. That's why, when there are several horses in a field, there are always some awake - on look-out - while the others cat-nap.
When a horse is transported by road, the frequent changes of speed and direction (going round corners) mean that the animal has to keep rebalancing itself.
This can be tiring on a long journey, and also makes it difficult to snatch a few minutes sleep every so often.
Air travel is smoother, and a horse will arrive at its destination having been able to sleep naturally - unlike its human team members who have been crammed into upright seats.
So a horse will not arrive weary merely because of the flight.
What about the time zone effect?
Because it distributes its sleep throughout the day, a horse won't necessarily feel sleepy at the wrong time of day. But the body clock is not just about sleeping.
Temperature and levels of hormones such as melatonin fluctuate on a roughly 24-hour cycle, in horses as well as humans.
In a recent study Dr Barbara Murphy and colleagues at the University of Kentucky at Lexington engineered a sudden six-hour shift in the 'day/night' cycle of horses kept under artificial light.
They found that the horses' melatonin levels adjusted to the new rhythm after only 24 hours, whereas body temperature was back in synch after three days.
Thus, they concluded, horses recover from jet lag quicker than humans.
Speaking from Cincinnati before the 2010 World Equestrian Games, John McEwen, the Team GB Head Vet who is also chair of the veterinary committee of the sports governing body (FEI), explained that he does not see jet lag as a particular problem.
"Our horses are athletes. They arrive at the airport before flight in tip-top condition. Most travel two to a pallet, facing forward, like two horses in a trailer. Their grooms travel with them, and adjust their feed pattern gradually.
"The pilots use every inch of runway, so there is no slamming on of brakes when we land. There is a 42 hour quarantine period, after which we begin exercise and most horses are competition-ready in about a week."
David Rothery is a Senior Lecturer in the Open University Faculty of Science. He is a volcanologist and planetary scientist, but has his own horse which he commonly takes on half-hour trips by lorry to lay the trail or otherwise participate in events organised by his local hunt. He is an academic consultant for the BBC Bang Goes the Theory series.