Nonentrained Circadian Rhythm Sleep Disorder:
Some people have a circadian rhythm that is misaligned from the natural day-night pattern. Usually, these people cycle between periods of insomnia at night and excessive sleepiness during the day. Everyone has a genetically determined day-night cycle (called tau) that is not quite 24 hours. If you lose external cues, such as light perception in the blind, your body will follow this genetic rhythm.
In general, this endogenous or innate rhythm is not quite 24 hours, either too short or too long. Therefore, your schedule may be off 30 minutes every day. Over the course of weeks, this time adds up and you may have your sense of night and day completed shifted from where it should be. Once the phases are completely reversed, you will have terrible insomnia at night and excessive daytime sleepiness as you desire to sleep in the day. Gradually, this will shift back to a more normal pattern, only to change again over a few weeks as it keeps shifting.
This disorder results when the light input to part of the brain called the suprachiasmatic nucleus becomes lost. Without this input, the body cannot become attuned to the natural circadian rhythms. Fortunately, it can often be effectively treated with low doses of melatonin.
Example: Sarah is a 34-year-old blind woman who has no perception of light and suffers from periodic bouts of insomnia and excessive daytime sleepiness. Every six weeks she cannot sleep at night and spends the entire day miserable and profoundly sleepy. Her family notices that this seems to be a gradual change that tends to correct itself, only to recur again.
Learn more about circadian rhythm disorders.
Source:
American Academy of Sleep Medicine. "International classification of sleep disorders: Diagnostic and coding manual." 2nd ed. 2005.
