Jet lag can occur when sleep-wake patterns are disturbed. A person may feel drowsy, tired, irritable, lethargic, and slightly disoriented. It can result from traveling across time zones or from doing shift work. The more time zones a person crosses in a short period, the more severe the symptoms are likely to be.

Jet Lag can affect people differently.  Older people may struggle more with jet lag than younger people and how you handle it may be different than how someone else does.

Consider the example of flying from Denver to London where the time difference is 7 hours.  London is 7 hours ahead of Denver, so noon in Denver is 7p in London the same day.  A flight from Denver will take about 10 hours to reach London.  If you depart Denver at Noon (7p London time) and fly for 10 hours, you will arrive in London at 10p 'your time', which is actually now 5a London time the next day.  As you departed Denver at Noon, you probably didn't sleep much during the flight.  When you arrive in London, it's the start of their day, but you are ready for bed.

So what can you do to help beat jet lag?

You could check-in to your hotel and sleep through the day, only to wake up in the evening rested, but you are now awake the entire night.  Another choice is to force yourself to stay awake as long as you can during the day and try to go to bed at a normal hour in the evening.  From experience, staying awake is usually the better choice.  Although it can be difficult, staying awake will help your body and sleep pattern adjust to the local time.

To help stay awake though the entire day, pick activities where you are active.  I would not recommend attending a symphony, play or lecture where you are going to be sitting and not moving around.  Get outside if you can and go for a walk.  You will find the more active you are, the easier it will be to make it through the day.

 You may also want to avoid alcohol as it may tend to put you to sleep.  If you are going to drink, wait until later in the evening or shortly before you plan on going to bed to enjoy your drink.

It may help to sleep during your flight if possible, so you could consider a prescription for a sleep aid such as Ambien or some other medication.  Ambien is a common sleep aid medication for travelers, but it does require a prescription.  As always, be sure to consult with your doctor before taking any medication.  For flights to Europe, I personally do not take it, but have taken it on longer flights such as the 14-hour flight from Canada to India.