Not necessarily when you normally go to sleep, but the time when you find it's easiest for you to get some rest. For me, it often seems like the only time I can get to sleep without a sleeping aid is when I stay up the entire night and fall asleep just before dawn. That's usually around 4:30 AM or so. Any later and the light from the sun will distract me, any earlier and I'll usually just lay around in bed for hours with my eyes closed. That's otherwise pretty much the only time I can get to sleep without hassle without taking something like melatonin.