When I had classes at 8am, I was usually in bed no later than 12am to wake up at 6. Nowadays, when I have few or no obligations, I just go to bed whenever I feel really tired and let myself naturally wake up. For a while I was consistently sleeping around 11:30-12, but lately I've been sleeping later and have been developing bad sleep habits. For instance, it is currently 4:15am and while the lights are off and I'm cozy in bed, I'm on my laptop still and haven't "gone to bed" yet.