I guess you have a pretty regular sleep schedule… I don’t. I have issues sleeping, so sometimes I would go to bed at 2am and fall asleep at 4am, other days I would go to bed at midnight and fall asleep immediately. I sometimes wake up at 7am when I have to go to the office, sometimes at 9am, right before the morning meeting when I work from home. During the weekend I wake up between 10am and 2pm… I also frequently wake up during the night, I don’t have a very deep sleep. So being in bed for 6 hours might only mean 5h of sleep. I am a zombie when I wake up, so I would most likely forget to write/mark down the times. It’s just not feasible for everyone to do that manually.
To answer OP, I use my Fitbit to track my sleep. Before I got my watch, I used Sleep for Android.