Sleep patterns and the amount one can sleep are all normal casualties as one gets older. In my case, my ability to sleep has diminished to a point where I can only sleep around 4 or 5 hours at a time before I wake up. This led to developing a pattern of napping once or twice during the day for around 20 minutes which seems to be sufficient.
But over the few months a really annoying version of the pattern has developed. With the coming of the summer heat, I've been getting up at 4:30 in order to get out hiking around 7:00 to beat the heat. On these days I will be in an extremely sound sleep when the alarm goes off usually with me feeling like I could have slept much more if only I had not needed to get up.
On the other hand on the days when I CAN sleep as late as I like, I am ALWAYS up much earlier than I have to be. Sometimes as early as 3:30 AM. I can almost never fall back into a deep sleep afterward.
It is exceedingly frustrating.