For some reason the days in the summer seem so much longer than those in the winter. I think it might be because it's lighter out so much later than it is in the winter. Where I live, in the summer it's bright and sunny until 8:30pm or a bit later. In the winter, you're lucky to see any sunshine and it's always dark by 4:30pm, 5:00pm at the latest.
It's not because my routine changes when the seasons change. I get up at 5:00am and go to bed most nights around 10:00pm six days a week year round. (Sundays I sleep in until 7:00am. It's a luxury I refuse to give up.) I seem to get so much more done in the summer than I do in the winter.
What about you? Do you find you get more done in the summer than the winter?