Sitting still on a jet for an 11, 14, or even 16-hour flight can be very stressful. Just like running a marathon, it's something you have to train and plan for.
It's also kind of like spending the day in the desert, miles from civilization, because although there are a few things your flight attendant will bring you if needed, it's not like you can pop down to the corner store for anything you forgot to bring.
You'll mostly be left to survive using your wits and your carry-on bag. So it pays to be prepared.
Assemble your life support system
There are lots of must-have items for your carry-on bag, but seriously, what you put in that bag is what you have to keep you comfortable, sane, and alive for the next half-day or more. You don't want to be stuck halfway through a 13-hour flight without a product or medication that you normally rely on. Most importantly, pack extras of any prescription medications and over-the-counter drugs you might need.
Ensure you have the right kind of headphones and adapters
If you have a late model iPhone, you probably carry lightning plug earbuds that won't work on the headphone jack in your seat, making the in-flight movie a disappointingly silent experience. Same goes for Bluetooth headphones. Some airlines still hand out headphones, but not all. To be safe, pack a pair of earbuds with the old plug, or a Bluetooth transmitter that can plug into the plane's jack.
Figure out if you can get Wi-Fi
Some airlines offer free Wi-Fi, others offer it for a fee, and still. Whatever the case may be, you should make provision for your own Wi-Fi so that you won't feel bored especially if there is no free internet.
Make a work plan
If you plan to work on the flight, download any documents you might need since you can't count on that in-flight internet. Make sure your laptop is fully charged, and bring an extra battery pack or a portable charger. If you use a wireless mouse, pack extra batteries for it. If your work is confidential in nature, a laptop privacy screen may be necessary.
Make a backup entertainment plan
Don't just assume you will watch movies provided by the airline. First, your plane might not even be equipped with personal screens. Second, the entertainment system or Wi-Fi may fail. Third, the available movies might not appeal to you. Download TV episodes or a movie onto your laptop, tablet, or phone. If you have a Netflix subscription, you can download some shows and movies there. You can also download books and audiobooks. Bring a portable battery pack for the device you'll be using for entertainment.
There is something about the physics of air travel that sucks every molecule of moisture out of your body. I know it's tempting to not drink anything before a flight, to avoid having to get up and use the bathroom, but you've really gotta drink water — at home, at the airport, and on the plane.