"Fixed" daily rhythm
For years I've had pretty constant sleeping rhythm. Even suring weekends and vacations I tend to wake up relatively early even if my bed time can vary. I just don't seem to be able to sleep past certain time except in really special conditions.
But recently I have noticed the rhythm have been shifting for a bit later. Where earlier I was up by 6 o'clock the latest now it has drifted towards seven when I tend to wake up. And I don't think I usually stay up that much longer than before.