Lots of good advice in this thread already, but I want to add a few details:

• that loose hoodie would be even better if the pockets zip up. Losing important items on a plane sucks. Also the pockets work great as a security checkpoint stash: remove your metal and empty your pants pockets all into your sweatshirt pockets, zip them, remove and put in the bin. On the other side you put it back on and have everything easily available.
• if you can’t find a zippable hoodie, go with a non-hoodie with zip pockets and a beanie that you can pull down over your eyes.
• for sleep aids, it’s a good idea to wait until after the first meal, which is usually within the first two hours of the flight, but you can ask and make sure.
• make sure your headphones have a plug, not just Bluetooth. The in-flight entertainment usually does not work with only Bluetooth, though some do.