1. For tracking purposes, I assume that I fall asleep immediately after putting my phone down. That's generally true enough, although sometimes I do find myself still lying awake and having to add a "Not sleeping" entry before putting my phone down again (for hopefully the last time of the night)! After my alarm goes off or as soon as I'm otherwise conscious enough to do anything, I mark "Sleep" as over.

2. It's not surprising that lack of sleep can get you feeling down. But since starting to track myself, I've been more conscious of the fact that whenever I'm feeling down, it's almost always after a night or two of bad sleep! I now take sleep a lot more seriously and ask myself first whether I've been sleeping well whenever I'm feeling down (as opposed to blaming other people, etc). This is especially important since I tend to ignore what my body tries to tell me.

3. The 7.5 hour average has remained roughly constant month over month for the entire decade. It's hard to say how much of an effect that other variables like exercise have since I always aim for 7-8 hours regardless. I do notice that if I get like 4 hours one night, I'll end up sleeping more than usual for the next night or two.