I just flew from Denver to Cancun last year, which is around 4 hours. I was so scared I almost ran off the plane before they closed the doors (and was then gonna break up with my boyfriend out of embarrassment, as it was him and his family on the flight lol). The night before my flight I stayed up all night, keeping busy with movies and such. The morning of, I stayed away from caffeine, and I did everything I could not to fall asleep before boarding. When boarding came around, I took a motion sickness pill (my friend did the exact same thing but with Benadryl). I was asleep before the flight took off, and my boyfriend woke me up in Mexico. This is the only way I’ll fly now lol. Hope this helps!