My wife is basically able to go to bed any time after we have finished eating dinner. I cannot go to bed until it is dark out. At this time of the year that means I too can go to bed any time after we have finished eating dinner. However my bedtime is much, much later in the summer.
I think this stems back to my childhood. When I was I am going to say somewhere between 8 and 10, I had a paper route (actually I had 4 they rolled into 1). Every Wednesday, that meant that my mom and I were up and out of the house by 5:30 AM to deliver papers.
This was not that big of a deal in the winter when I was in bed early and it was already dark out. But it was not that much fun in the summer when I was in bed not only when it was still light out, but when I could hear my friends still outside playing. I guess that I can't go to sleep until it is dark out now because I feel like I might be missing out on something.