An unbalanced circadian rhythm can be returned to equilibrium through the application of light to a sleeper’s retina near the end of a person’s “internal night.” Internal night? Yes — it may be night outside, but if your circadian clock is not prepared for sleep, internal night may not start until late and last well into morning. Biologically, it coincides with the secretion of melatonin by the brain’s pineal gland. It is difficult to know where your internal night lies if you artificially force sleep earlier, for example with sleeping pills. You can estimate internal night with a quick chronotype questionnaire that helps determine when light exposure will be most effective for syncing your circadian rhythm with external reality.
Seems like my natural bedtime is right around midnight. I do notice that I sleep better on days when I spend time outdoors, though maybe that’s tied up with better eating or moderate physical activity, too.
Sleeping (or Not) by the Wrong Clock – Opinionator Blog – NYTimes.com
Mark Larson 