We've looked forward to the vacation for weeks, even months. We've planned, we've packed. And then,  after the flight to, say, Europe, we're beat. We walk around in a jetlagged fog for days, unable to really enjoy the destination.

It doesn't have to be that way, says sleep expert Karl Doghramji, director of the Jefferson Sleep Disorder Center.  With vacation season approaching, we spoke to him recently about how to ward off the worst of it.

Physiologically, what is jet lag? Can even short flights cause it?
Jet lag is a complaint of fatigue, sleepiness or poor sleep that accompanies jet travel. It's caused, we think, by a mismatch between our internal biological clock and our environment's light/dark cycle.  Because of a rapid east or west flight, we're in a new time zone, but our biological clock is not. It's back in the old time zone. Thankfully, our clocks do adapt. However, it takes a little while.

The symptoms include a host of other things. Because of fatigue, people have poor concentration, a sense of malaise, and tiredness; jet lag is also associated with joint aches and gastrointestinal disturbances like constipation or stomach aches. Headaches are not uncommon.  If they're on a business trip, they may fall asleep during meetings. If they're on vacation, they may not enjoy it as much. They're looking at the Eiffel Tower, and they feel like garbage.

One time zone change, in general, is unlikely to bother people that much. The more time zones you cross, the worse it gets. If people have medical problems to begin with — if they're already not sleeping well — they're  vulnerable.  If people have certain sleep disorders, like sleep apnea or narcolepsy, they also are less likely to adapt or adapt less rapidly.  Also, unfortunately for retirees who now have the time to travel, the older you get, the more difficult it is to adapt. The good news is that retirees can take longer trips, staying in destinations for longer, which makes it easier.

Which direction of travel – east or west – is worse?
For most people, traveling east is harder to tolerate. Most of us have an intrinsic biological period, also called a day length, of a little longer than 24 hours. If I were to put someone in a cave, they would live a day length of about 24.2 hours. If that person travels west, their body is already delaying a little bit naturally. On the other hand, about 20 percent of people have a less-than-24-hour clock. In their cases, traveling east may be easier.

Another interesting tidbit: Some of us are owls and some of us are larks. Owls do well as the day progresses. They have their maximum alertness in the evening. They stay awake until 2 a.m. and  sometimes have difficulty getting up in the morning and going to work. The larks just naturally wake up early, and they're ready to go. But come afternoon, they're flagging. By 8 at night, they're ready to go to bed.

Owls have an easier time traveling west. Their day is already a bit delayed to begin with, so when they get to San Francisco, they fit right in.  But when they go east, to Frankfurt, they have to force their bodies to get up even earlier than they normally would.

The larks, on the other hand, adapt more rapidly when they go east. They're already getting up early,  internally. They have greater difficulty going west.

What can travelers do?
The best way to cure jet lag is to prevent it.

Let's pretend we're traveling east.  The advice I give people is to start three or four days before traveling, and go to bed an hour earlier every day. Wake up an hour earlier. By the time you get to Frankfurt, you're almost right on time.  And before you go to bed, for about two hours wear glasses that block the blue light from coming into your eyes.  They're yellow in color, but they filter out blue light. You don't need a prescription; they're commercially available.  Also, it's best to make things a little more dim in the room to prevent light from coming into your eyes for a few hours prior to bedtime. You'll fall asleep more easily.

In the morning, as you're getting up earlier by one hour, expose your eyes to bright light, to start pushing your sleep rhythm in the right direction in anticipation of eastward travel.There are commercial lights that provide 10,000 LUX – a measure of illumination. Use one of those for about half an  hour. Don't look directly at the light. Sit in front of it as you read or watch TV so you have the light in your peripheral vision. (Before using any lightboxes, check with your doctor, who will determine if this is medically safe for you.  It might not be for those who have mood disorders or eye disorders like glaucoma, or if you're taking certain medications. Some people cannot tolerate bright light.)

Not everyone can do all this. But the more you can do, the better off you'll be.

What about during the flight?
If you're heading to Europe, it's usually an overnight flight.  Don't drink alcohol or caffeine on the flight. Those can worsen sleep. Wear the eye shades the airlines often provide to prevent light from coming into your eyes. And wear ear plugs or the headphones that block out noise. Get as much sleep on the flight as you can. If you can take a direct flight, that's better.

I also generally recommend melatonin.  It helps with sleep. Check with your doctor, but it's available over the counter. Try it for a few nights before you leave. If you're doing okay — no side effects — take it at bedtime for a few nights before you leave, and the entire time you're in Europe.

After you arrive, immediately start living the local sleep/wake time.  Don't try to fool the system by sleeping Philly time while in Frankfurt.  Don't go to bed even if you feel tired.  That afternoon, take a brief nap, about half an hour, just enough to pay back that sleep debt, but not long enough to disturb your new sleep/wake rhythm.  In the evening, use those blue light glasses before bed, and in the morning, get a lot of light.

As for coming home?
It's going to be easier coming back.  Typically, the flights back from Europe are day flights.  When you come back, don't fool your body by going to bed Frankfurt time. Try to go to bed Philly time. Don't come back on Sunday expecting to go to work on Monday. Give yourself a day or two to adapt.