Your opinion is fine, but almost certainly wrong. Everyone has a natural cycle. I think the problem is that we have so many problems with our sleep that are even greater than the impacts of the differences between one cycle or another.
I just think our bodies are able to adjust to the inputs we give it. It's difficult, no doubt, to shift the schedule our body gets used to sleeping on. But with discipline, I think anyone can shift the schedule from early to late or vice versa.
If you're saying we all have a natural 'early' or 'late' cycle, how would that even work biologically? What would determine that? Is it hardwired into the code of our DNA? Even if this is the case, you would be handed a certain natural 'cycle' from your parents. But we know that gene expression is not static. We know many gene expressions change fluidly throughout our life depending on our environment/ behavior.
So even if I was born with a proclivity to stay up late, I would still expect to be able to change that though concerted effort and practice.
I'm also probably a little conceited when it comes to sleep habits :) I feel lucky to not have any trouble falling and staying asleep.
I definitely have sympathy for people with sleep problems/ disorders. When I was in my early teens, I had horrible troubles going to sleep. I would lay in bed, sometimes hours, without being able to fall asleep. I grew out of it for whatever reason as I got older. This is interesting now that I think of it, considering the wikipedia someone posted below on chronotype, suggesting adolescents tend to prefer delayed sleep.
But the reasons adults or children have trouble with sleep are complex and difficult to identify I'm sure. I don't envy anyone with sleep problems.
Chronotype does have a significant heritable component. Studies have even identified specific markers near genes known to affect circadian rhythm.
Most people can shift their schedule up to an hour without too much trouble. They can also force themselves onto almost any schedule with strict habits and/or medication. That doesn't mean they adapt. One illness can wipe out months of routine.
Sleeping later is easier than sleeping earlier for most people. Bright light delays your sleep cycle.
Intuitively, it makes sense that people would have different sleep cycles. A pack of animals is more likely to survive if one is awake or sleeping lightly.
