As far back as I can remember, I always had trouble getting up in the morning... and no trouble staying up during the night.
During the years I went to school - and the brief time in my late teens when I actually worked in an office - having to get up in the morning was, without exception, horrendous. It was due to the experience of these years that I thought I hated mornings, and it took me a while to realize I really don't. I actually love being up and alert when the sun is rising, with the streets still calm as night, when nobody is yet up. I really don't mind getting up at that time, either. I just absolutely can't handle a 24-hour sleep cycle.
Early in my twenties, I started to become less dependent on other people's schedules, but I still tried to keep a 24-hour sleep cycle, because that's what you are supposed to do. I would still try to get up at about the same time every day, but the time kept slipping. Getting up at 10 am turned to getting up at 10:15, then 10:30, 10:45, and so on until I was finally getting up at 5 pm, and going to bed when the sun rose in the morning. I struggled against this tendency; I used alarms, I guilted myself to get up earlier, but I was never successfully able to turn the clock back. Always forward. When the cycle came around so I was getting up at 6 pm again, I pushed myself the following few days to stay up much longer, so that I would skip a day and begin waking up again at a reasonable hour. Always, I wished this time around I could start sticking to a reasonable schedule. But again, getting up at 7 am turned to 7:15... And then 7:30... And so on, until it was time to skip a day again.
I failed to stop the slippage at either end of sleep. If I tried forcing myself to get up at a consistent hour, this became increasingly difficult until my eyelids felt like cinder blocks, and it was impossible to honor the alarm. If I tried to make myself fall asleep at the same time each night, I would increasingly just toss and turn in bed.
A few months ago, I thought to myself, what the heck. I had always tried to keep a 24-hour sleep cycle. It had always been a struggle, and in vain. But by now, I'm no longer tied to nearly anyone else's schedule - and I'm far from anyone's judgment. So why not let myself stay up however long I want, until I feel like sleep; and then sleep for however long I need?
For the past few months, the result has been a close-to-25-hour sleep cycle. My wake-up time shifts by about 1 hour, on average, per day. For the first time in my life, I'm neither tossing and turning in bed, nor waging a "get up!" war between spirit and body. I'm rested and relaxed, and never groggy after I wake up. Throughout the day, I feel like I'm at full capacity. This is what a smooth experience feels like.
The negative, of course, is that for a good portion of the month, I'm sleeping through the day when other people would expect me to be up.
A known condition that fits these symptoms is Non-24-hour sleep-wake disorder. I have not yet sought a diagnosis, so I cannot claim that this is what I have. However, the article states: "The disorder is an invisible disability that can be 'extremely debilitating in that it is incompatible with most social and professional obligations'." That is approximately right. One could argue I'm fortunate to have a life and work situation that allows me to accommodate this condition. But the truth is, this is not coincidental. I have never been free of this tendency. I built my life and work situation around this. Chances are that I would not have made the same life choices if I didn't have what felt like a biological requirement to not get locked into a fixed work day.
It is a condition that limits. It does not help me to have it. I have worked my way around it, at some cost.