How do I know when I'm done sleeping?

Most of the time, I use an alarm clock to wake up every morning. There are no problems there because I simply KNOW that I have to get up at that time. But on the weekends, it's a different story. Take this morning for example. I woke up at around 6:30 to use the bathroom, but still felt tired so I went back to sleep. I woke up again at 8:15, and still felt tired so I went back to sleep. Then again, I woke up at 10:30 still felt groggy but I basically told myself to get up because I didn't want to sleep the day away. I notice on weekends when I have no definite wake up time, I will wake up every hour or so but still feel tired enough to fall back asleep. When I was younger, I remember on weekends waking up and feeling alert. That's how I knew to get up. Now that I'm in my late teens I feel like I don't have something in my body telling me when I'm alert. I sometimes think if I wanted to I could just keep going back to sleep. How do I get the feeling of alertness back so that I know when I have slept enough and I should wake up?