You know the feeling: it's three in the morning and you're walking round the house, glass in hand, perky as a lark and wondering who you can phone to talk to about the great time you had trekking through Laos. Or about the exotic delights of Bangkok airport. Or the marvellous attic apartment you stayed in the Bronx.
The next night, it's the same thing: while the rest of the house is dead to the world, you're still up, bright-eyed and bushy-tailed and it's two hours after midnight. The condition is known as jet lag and there's no surefire cure for it. Which is why everyone has their own tips and treatments, ranging from the dotty to the dangerous.
As always, prevention is better than cure. Getting in a couple of hours sleep before you travel is one, suggests Dr Graham Fry of Dublin's Tropical Medical Bureau (01-2857358). Planning your arrival dates so that you have a day in hand before you have to take important decisions also makes sense, he says. And, of course, getting some sleep during the flight.
But it's getting to sleep in the first place that is usually the problem for, although your internal clock is running on its own circadian rhythm, you've been moving in and out of time zones like tomorrow had already happened. And, added to this is the noise of a set of powerful jet engines thrumming away non-stop for maybe 14 hours. Stir in a mess of thick, recycled air of which there is never enough to go round and it's no wonder mind and body fall out of sync.
Some travellers swear by their own brand of sleeping pills and, with their help, manage to circle the earth without once shifting position even in the often-cramped conditions of economy class. However, not everyone wants to take sleeping pills, preferring, as one traveller said somewhat ominously, "to remain alert in case of an emergency".
What then are the alternatives? The Dublin-based homeopathic suppliers, Nelsons of Duke Street, (01-6790451) recommend arnica which jizzes up the circulatory system and prevents that feeling of over-tiredness that comes with jet lag. Nelsons' Travella - made from, among others, the Indian cockle plant - helps to prevent travel sickness while tincture of Avena sativa works on the nervous system and has a calming effect. This last won't knock you out but it should allow you to rest quietly during a flight and is certainly an alternative to the miniatures of brandy and bags of nuts that appear with the drinks trolley.
From the herbalists comes the interestingly named Human Bomb - a mixture of organic honey, ginseng, royal jelly and guarana. Available from the wholefood store Human Nature in London (0044 171 3285452) whose owner, Nari Sadhuram, recommends that you take one phial before landing and then one a morning for a week. A phial costs £1.99 sterling.
But the biggest thing of all which the jet-lagged body craves is sunlight, more so if the journey has been west to east, since it is then that you will have lost out on daylight. This is one reason, Graham Fry suggests, why you should spend as much time exposed to sunlight as you can after a flight. And this brings us to the increasingly popular nutritional supplement Melatonin, a substance which mimics the human hormone melatonin which is produced naturally in the pineal gland. Melatonin levels, manufactured by sunlight, go into action as the day darkens and tell the body to shut down for the night. If the melatonin levels are low - due to lack of sunlight - we fail to get the message and stay awake.
Melatonin, marketed in the US as a food supplement, has been approved by the Federal Drug Administration and is widely available in health-food shops there. It is not licensed in the UK, however, which means that though a doctor there may prescribe it - and some do - should anything go wrong, the doctor and not the manufacturer will be responsible. In Ireland, Melatonin is classified as a medicine and because not enough independent testing has been done on it to satisfy the Irish Medicines Boards - the statutory independent body which licenses such things - it has not been authorised for sale here.
Travellers, of course, bring it home from abroad. Many users advocate it as a useful antidote to jet lag and, apart from the US, you'll find Melatonin on sale at Hong Kong airport. Dr John Kelly, chief executive of the Irish Medicines Board, underlines the risks inherent in any treatment that has not been authorised by his board, especially the possible dangers to pregnant women or to people taking other medication.
Dr Larry Goodyer, a director of the London-based Nomad (0044 181 8897014 ) - which provides a comprehensive advisory service to individual travellers as well as to group expeditions - says that Melatonin will make you mildly sleepy which could be good if this helps you sleep during the flight. It may also be preferable to alcohol or sleeping pills as it won't leave the traveller feeling sluggish or with a hangover. Graham Fry's reservation about Melatonin is that frequent fliers might take it regularly, giving the body no time to adjust normally to the pressures being put upon it.
My own experience of Melatonin is inconclusive. After a long-haul return flight last year to Australia, I spent a good week trying to adjust. (The theory is that for every hour lost or gained while crossing a time zone, it takes one day for the body to return to normal.)
Determined my jet lag would not happen next time round, I took one Melatonin tablet during the flight and slept well for two hours (unusual for me) and spent another two hours resting quietly, arriving eventually at my destination feeling reasonably rested. Whether this was due to the Melatonin or to the fact that there was more room than usual on the plane is anyone's guess. The recommended treatment is from 3 mg to 15 mg of Melatonin to be taken around 9 p.m. for three days before flying. Continue for a few days after your arrival, depending on how many time zones you've passed through. Sixty tablets cost about £7. If you want to know more, you can order the book Melatonin, Nature's Sleeping Pill, from 04637317.