Another one who's used Tylenol PM reasonably successfully on overnight flights. I am a terrible plane sleeper, but the pills definitely help.
If you take something, I recommend taking it an hour or so before the flight so it has some time to kick in. The minute the flight takes off, put on your eyemask/earplugs and try to zonk out.
There's been some research showing the easiest time to get to sleep on a plane is just after it reaches altitude because there's a slight drop in oxygen levels due to air pressure and your body takes a little while to adjust. During that adjustment period is prime sleep-falling time. Acceleration also helps with a sleepy effect. Anecdotally I've most certainly found with my toddler that if I get her asleep right after take-off, she's good for hours...but if I miss that window, yeah, it's not gonna happen. (One source on this if it's of interest:
https://www.inverse.com/article/24244-why-fall-asleep-on-planes-oxygen-cabin-pressure-flight-aircraft)