People have a natural circadian rhythm (day-night, awake-asleep cycle) which is naturally slightly longer than 24 hours. In the normal world, our clocks get reset every day by the sunlight, so we keep fairly well synced up with the wall clock. Down here, the whole day-night thing only happens once a year, so we're more of less left to our own devices to work out a sleep schedule.
Since my arrival back in January, I've been having trouble hitting a regular schedule. For the first couple of months, I free-cycled. That is, I went to bed when I was tired, and I got up when I was done sleeping, time of day be damned.
Since sunset, that hasn't worked so well. Social opportunities through the winter are... limited, and after a couple of months away from civilization, quite necessary. Most of the crew works regular (7am-5pm) hours, so it's fairly important for mental health to stick to something at least vaguely resembling a normal work-play-sleep cycle.
I found out pretty quickly that without sunlight, it gets progressively easier to sleep longer and longer. After a 16 hour nap, it's pretty tough to go to bed again before putting in a 30-hour workday.
I've tried several options, but the only thing that really seems to work is to set my alarm to go off long before I finish sleeping, with the goal that I'll still be tired come evening. Since my infirmity last week, I've been trying this regularly, getting up at 6am daily, and its been a fair success so far. It's pretty tough to drag myself out of bed some mornings, especially knowing that there's no actual need for me to be up at that time, but on the whole, it's an improvement. (It also gives me an excuse to walk back and forth to DSL twice every day - before and after lunch - and delight in the amazing night sky.)
A lot of people get severe insomnia over the winter, only sleeping a couple of hours each night, and spending the days wandering around like zombies. All in all, I'm pretty lucky on the sleep front.