Those of us who are lucky to have functional eyes depend on them to align us to the most prominent circadian stimulus: the sun. Aside from the photoreceptors that help us to see, there are cells in the back of our eyes that project to a small cluster of cells in the brain called the suprachiasmatic nucleus. The SCN synchs our biological clock to light. However, folks that are blind (with some exceptions) are unable to synch to the sun. The result is that their biological rhythms “free run” according to their innate period. Unfortunately, this is usually slightly different from 24h, so their sleep and wake times drift away or towards that of the rest of society. Depending on how different their period is, they may only be synched with everyone else’s day-night cycle for just a couple days a month. You can imagine that if these people have regular daytime jobs, this is a nightmare situation. (To help your imagination: imagine having to go into work 20 min later each day while working the same number of hours per day, for your entire career. That’s what it feels like for some of them.)
If you’re curious about what your inherent period is, the only way to find out with certainty is to live in a constant environment for weeks without any external cues. No clock, phone, computer, or any other uncontrolled contact with the outside world. Sounds fun, doesn't it? But there are clues that don't require enduring such misery. If you're a morning person, this may be an indication that your period is slightly less than 24h, and the opposite for night owls. (See my previous post on morning and night people.)
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