Slight hypoxia, like on an airplane, can signal nighttime, and becoming normoxic upon exiting can serve as an alerting signal.

“The other thing that will help is planes are slightly hypoxic of an environment and oxygen rhythms actually there's some great data um there's there there's a there's a lab in Israel that's been working on this especially showing oxygen itself is a circadian signal because oxygen dips during the night when you're asleep. So being in a slightly hypoxic environment for an extended period of time, it's easier to trick your body into thinking it's nighttime than it's daytime. And then as soon as you leave the plane and they open the door and now it's normoxic again, that can be an alerting signal.”