Whenever I'm tired. Currently, that happens every 30-40 hours. It's a little confusing to wake up on Monday, go to bed on Tuesday and sleep until Wednesday, but one gets used to it. If I happen to be awake during the day, I do my shopping. If I'm up during the night, I do household chores. But most of the time I just sit around, so it doesn't really matter if it's night or day, weekday or weekend, summer or winter. And the fact that I never know which time or day it will be when I wake up (or, on rare occasions, which year) keeps my otherwise boring life mildly interesting.