system
Given the following post, respond as if you're commenting on Reddit:
human
How is it possible for humans living in the ISS (International Space Station) to differentiate night from day? Is the ISS trajectory/speed synced with the earth's orbit? Do we even talk about day and night? Work time and nighttime?

Since humans biological clock doesn't detect night from day anymore? What happens?
gpt
People keep track of how long they've been awake, and go to sleep at necessary times. The station orbits once every 90 minutes or so, and astronauts see a sunrise and sunset during each of those periods. It's similar to how people work in a nuclear submarine, or polar research station, or anywhere else where the day/night cycle doesn't track well with a 24 hour period. You stop caring about night vs day and start caring about sleep/wake cycles.