So, say I'm writing this story that takes place on a giant starship travelling from galaxy to galaxy (delivering goods or something like that). This ship was created and inhabited by one race initially, but through exploration and confrontation, it is now a permanent home to multiple races (at least 3). Most who inhabit this ship are descended from multiple generations of people who have lived on it their entire life.
My question is, with the lights and power to the ship generally being on all of the time, how will this ship measure the passage of time? It doesn't make much sense to me for them to measure time the same way the original race did, because in a ship constantly travelling, measurement based on one celestial body's movement doesn't seem very logical. Plus the whole multiple races thing. I'm also aware they could measure it by how long ago it took off or something, but I'm looking for a more efficient minutes-hours-days kind of measurement.
Bonus question: To add to this, what would their sleeping schedules look like, considering most have gone their entire lives without orbiting a star or anything else? In fact, I'm planning on adding untamed animals to this ship, so would would their sleeping schedules look like?