What is really interesting is that human's natural sleep cycle is not to sleep continuously for 8 hours. Our natural sleep cycle is to go to bed when it gets dark and then to wake up for a few hours in the middle of the night and get up and do things (apparently this is also the time when most sex happened since other family members in the same room were still asleep) and then go back to sleep and get back up at dawn.

Does this mean the natural sleep cycle time is a lot less in summer than it is in winter?

Why would other family members had been asleep? Wouldn't this natural sleep cycle have applied to everyone?

I live at 60 degrees Northern latitude, and I sleep a lot more in the winter. And it's really difficult in the summer too, because it never gets dark and you need to go to bed to get up in the morning.

For some reason it's not difficult for me to sleep past sunrise, but very hard to go to bed before sunset or wake before sunrise. And some people are the opposite.

This just applies to adults. Children and teenagers naturally sleep longer than adults and probably slept throughout the night. Plus, apparently people didn't all get up at the same time during the night. Some would be up from midnight-2am, others from 2am-4am, etc.

For those landing on this comment, some reference: http://slumberwise.com/science/your-ancestors-didnt-sleep-li...

For the most part i agree, but how about summer times in the northern regions in US or the world when sun doesnt set until beyond 9pm or say Norway where it can be until midnight or more than 60 days in some areas?

If you mean day/night, that's the rotation of the Earth around its axis, not the rotation around the Sun.