My grandfather did it after he retired, but that’s because he farmed for years (and farmers get up early, in my experience).
I would guess it’s a more natural sleep pattern: to space the hours more evenly around midnight, to fit the hours of darkness.
If you think about it, are working lives are very much separated from the pattern of natural light. A lot of people work 9am-5pm, or increasingly nowadays, 9.30am-6pm. I don’t start work until 10.30am and finish between 7pm and 11pm, depending on the day.
So most people work almost twice as many hours after noon as before. That means they tend to go to bed later and get up later. I doubt if many working people get more than an hour or so of sleep before midnight on a regular basis.
Once you retire, your sleep patterns can go back to fitting in with daylight a bit better.
I’d far rather be up at 11 PM, as a matter of fact.
It’s 6 AM now, I’m going to go read ATMB one more time and then I’m off to bed.
I was always a night owl, even as a child. I worked at night for years and years, it was a much more natural time for me, I was alert and lively. It didn’t seem to matter how much sleep I had, once the sun went down I got a big burst of energy.
But once I hit menopause it all changed. Suddenly, I was up with the sun, for no good reason. Crazy. But, at least I was sleeping through the night. I knew women who couldn’t manage that! The flipside of that is, if you were up with the sun you’ll feel like retiring early. I have a hard time staying up till 11:30pm some nights, now. What a turn around.
It is a still and beautiful world, dark, snowy and quiet, just as the sun rises, on a winter morning. Building a fire while the dog still sleeps, watching the snow blow, it’s hushed, in a way. I’m surprised to find a part of the day I’d entirely missed, for so many years!
ISTR reading here on the Dope that older folks need just as much sleep as anybody else. However, as we age, our brains stop producing as much melatonin, the hormone necessary to keep you asleep all night. This results in waking up in the wee hours (to wee! heh) and not being able to drop off again. After awhile one gives up and gets up, cursing, and this results in fatigue and wanting to go to bed earlier that night. And the vicious cycle continues.
The reason you get up early is to go to work. Starting at 7 am will get you up early.
When you retire ,you do not have to get up early anymore. You wake up when you feel like it. At 6 AM ,I don’t feel like it.
That’s not true. I’m 72 and retired at 65, but I get up earlier now than when I worked. Several reasons. On Tuesdays, Thursdays, and Saturdays we play tennis in the morning, starting before 7. I get up before 5 on those days for several reasons. My cat, Wilson, who sleeps with me, wants me to get up and clean his litter box (although sometimes he puts his paws on my legs so I won’t get up). The neighborhood cat, Harry, wants to come in around that time to nip at my cat nip and get a snack before he seeks nourishment elsewhere. I have to do my daily calisthentics, make breakfast, etc. before tennis. On Sundays, I meet a group to run at 6:30. On Wednesdays, I meet a group to bird watch at 8. That leaves Mondays and Fridays. But since I get up at 5 the other days, I also get up at 5 on Mondays. Hey, I have to let Harry in for his catnip and snack.
I also go to bed early: after the seven o’clock evening news, if ETV doesn’t have Sherlock Holmes (Saturday evenings at 8), or NatureScene (Wednesday evenings at 7:30). Sometimes I’ll stay up later to do a little reading, but after my two glasses of wine with dinner, I’m sleepy, and, besides, Wilson wants me to go to bed. So, I get 8 or 9 hours of sleep. I will get up once or twice to use the washroom. (As we age, our bladders shrink. BPH can cause this, too, but since my laser surgery, I don’t think that’s the problem in my case.)
I find if I do sleep later for some reason (no tennis because of rain), I’ve wasted time sleeping, and by the time I go to the Wellness Center, run, bike, do the weights, I may run into the evening rush hour. Today, for example, heavy rain was predicted (it finally arrived in the afternoon), so I went to the Wellness Center around 6. That way I beat the crowd and the rain.
Barbitu8 puts us all to shame as well. You guys rock!
Somewhat related to the OP, this report on sleep habits as people age, came out a few days ago. Although the research showed that older people get less sleep, I’m not sure the claim that “(the elderly) need less sleep” follows. But, then again, I’m sleeping less nowadays and am tired during the day - so I may have been too muddled to understand. 
Here’s the abstract to the article in the journal Sleep.