The researcher can precisely determine, for example, the daily cycles of gene activity, body temperature, blood pressure, hormone secretion or sleep and activity/alertness.
[2] The effect of unintended time cues is called masking and can totally confound experimental results.
The many blind people who do entrain to the 24-hour light/dark cycle have eyes with functioning retinas including operative non-visual light-sensitive cells, ipRGCs.
[5] These ganglion cells, which contain melanopsin, convey their signals to the "circadian clock" via the retinohypothalamic tract (branching off from the optic nerve), linking the retina to the pineal gland.
As with delayed sleep phase disorder (DSPS or DSPD), in the absence of neurological damage due to trauma or stroke, cases almost never appear after the age of 30.