Seems like my experience is different from almost everyone in that when I was younger (teens and early-mid twenties) I did horribly on less than 7 hours a night. I tried my hardest to get 8-10 hours. If I got 6 or under I was not fun at all to be around. I also had bouts with insomnia, so I never had classes or jobs where I had to be up before noon.
For my first semester in college I was getting 4-5 hours a night and was getting serious sleep deprivation (constantly falling asleep in classes, HAD to have an hour or two nap in the afternoons, etc).
Now it's totally different. 7 hours of sleep in a night is A LOT. Every once in a while (once a month or so) I'll get in a night of 12-14 hours, but 4.5-6 is much more common.
I'm also finding the quality of the sleep to be a much more important factor in terms of how I feel during the day. If I have a really bad night of sleep, a 15 minute nap mid-day can turn the whole day around for me.
Another major factor was getting my overall health under control. I've lost a ton of weight the past couple years and am eating about 1,000 times cleaner than I used to. I haven't really thought about it before but I suppose that could be impacting my sleep and almost complete lack of insomnia/asthma symptoms now as well.