We're a culture in perpetual motion, so it's hard to always have the same sleep schedule, day in and day out. Either it's travel that messes up our schedule, or the variations in morning and evening schedules that come from parenthood, or we might work jobs that don't support a regular sleep schedule.
So if forced to choose, which is better? Going to bed at the same time at night or getting up at the same time every morning?
Circadian resetting occurs at the time you awaken. What this means is that the morning light sends a signal through both your eyes and--believe it or not--your skin, which stimulates your wakefulness drive. If you get up at the same time of day, even as seasons shift, your body resets to the circadian rhythms of the planet, which more or less guide the processes of all living things. It's easier to stay on track if you get up at the same time every day. It's even better to GO OUTSIDE for even 10 minutes, as that short dose of sunlight (even if it's dim or overcast) is still brighter than the lights in your house, and this will help you fight sleepiness during the day.
At night, it's a good idea to try to stick to a bedtime schedule (especially for children or for people with challenging schedules where they have to work hard to make sure they get enough sleep between shifts). But sometimes you just aren't as sleepy at that time. No worries. Remember that wakefulness drive I mentioned earlier? It works in tandem with your sleep drive. We all have a built in drive to sleep (just like we have a hunger drive, or a sex drive, or a thirst drive). That drive is informed by how much we slept previously (that includes naps during the day). So if you normally go to bed at 9pm and get up at 5am, and you take a 2-hour nap at 1pm, you might not feel as sleepy later, at 9pm, because you actually banked a couple of hours of sleep, thus delaying your sleep drive. So what do you do? You just go to bed when you begin to feel drowsy and don't worry so much about what time the clock says. Your body has its own built-in clocks, it will let you know when it's time to hit the hay.