Reading this book about biological clocks, and picked up these fun facts:
1) there are light receptors in the eye that apparently are never used for vision: their function is to inform the clock center (there is one) about the amount of light. Blind people may or may not still have the use of these receptors: if they don't their biological clock won't synchronize with day and night properly. If they do, they synchronize as well as sighted people.
2) It's a LOT brighter outdoors than indoors. Even on a dark day, lots more light showering down outside than in a brightly-lit room. The numbers really surprised me. If you're having trouble synchronizing your clock with the day and night cycle, getting outside as soon as you wake up will probably do you a lot more good than dimming the lights before bedtime.
3) People vary, but most commonly, if kept in a space with nothing to reveal what time of day it is, most people will settle to a circadian cycle longer than 24 hours. So the system *tends* to run late. If you don't keep nudging the clock by getting that blast of light in the morning, the natural tendency will be to stay up a bit later every night.