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.
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.