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Opt for the most comfortable one you can afford.  Many airlines offer fully reclining seats that convert into flat beds in first class and sleeping berths with beds are often available in the first class compartments of trains.  If first class travel is outside your budget, think about paying for extra leg room on a flight, especially if you are taller than average.  Even if you're stuck in coach however, you can still get a seat that works for sleeping.  Get a seat that reclines.  If traveling by airplane, avoid the back row of the plane where the seats often don't recline fully.  On overnight train journeys, reclining seats with leg rests are usually available in coach class. Pick somewhere quiet.  On planes this is typically the middle rows where you are less likely to be distracted by the noise and movement of other passengers and flight attendants. If you find that being near children can disturb your sleep, avoid bulkhead seats on a plane as this is often where the cribs are located. Go for a window seat.  Many passengers report that it is easier to drift off in a window seat where you can rest your head against the side of the plane or train. You are also less likely to be disturbed by another passengers getting out to the aisle. Get a good bunk on the train. On longer European train journeys, shared couchette accommodation is popular and affordable.  A couchette is a room with several bunks, which you will share with strangers.  The rooms lock from the inside.  The top bunk is generally more secure, but if you need to get up during the night to use the bathroom it can be difficult to do so in the dark. This is more pertinent to train travel because although trains are generally very safe places, robberies do happen.  When you're sleeping you are more vulnerable, so prepare for the worst by keeping valuables with you at all times. If you feel comfortable and safe it will be much easier to get some rest.  Consider using a money belt that straps to your waist or thigh. If you are storing luggage in the overhead bins, make sure the opening is facing away from the aisle and consider using a lock. Packing these items in your hand luggage will help you create the right conditions for sleep.  An eye mask.  Many airlines give these out for free when you board but they are not generally available on trains.  Opt for an eye mask with a flap that comes down over the top of your nose to keep the light out as much as possible. Earplugs.  Airplanes and trains can be very loud places, with the sounds of other passengers, the meal and drink service and plane engine noise potentially disrupting your rest.  Bring foam or silicone earplugs so that you can block out the noise and get some sleep. As you settle into your seat, tell your neighbor that you are planning on sleeping.  On a plane, let the flight attendants know that you don't need to be woken up for meals or drinks. Set it for an hour before you are scheduled to arrive at your destination. Remember that you may be crossing several time zones and be aware of the local time at your destination.  Feeling prepared and ready for what's ahead will help you to relax and get some sleep without worrying about what will happen at your destination.
Choose your seat or bunk carefully. Secure your belongings. Bring the necessary equipment to encourage sleep. Communicate to avoid being woken up. Set an alarm.