FoundationalConceptsNeuroscience

Now suppose you either leave the lights on all the time in the rat’s
cage or turn off the lights and leave them off all the time, exposing the
animal to either constant light or constant darkness. You can continue
this experimental arrangement for many days, or weeks, or months.
Under these conditions the rat will continue to alternate between
cycles of activity and inactivity with an approximate period of twelve
hours of activity followed by twelve hours of inactivity—that is, a
circadian rhythm of activity or sleep-wake is maintained. However,
the overall period is generally not exactly twenty-four hours, often
being a little longer or a little shorter. This is called the “free-running”
period of the circadian clock, in that it is decoupled from synchroniza-
tion by environmental cues (like day and night) and so “runs free,” de-
pendent only on internal mechanisms for keeping time.
Analogous studies have been done with humans who volunteered
to live in caves and bunkers, completely shielded from any outside
light. They were allowed to sleep when they wanted and be awake
when they wanted. They could turn lights off and on when they
wanted and eat when they wanted. The experiment was maintained
for several weeks. Under these conditions, the participants tended to
develop regular schedules of activity and sleep. Although their sched-
ules of activity had periodicities of around twenty-four hours, they
often were a little longer—folks tended to wake up a little later each
day and go to bed a little later each day. It is as if their endogenous bi-
ological clock had a free-running period slightly longer than twenty-
four hours.
Many studies such as these have suggested that animals and other
organisms have endogenous biological clocks that are circadian—that
is, generate a periodicity of approximately twenty-four hours. Under
natural conditions, this periodicity is synchronized by environmental
cues to bring the ticking of the endogenous clock into alignment with