FoundationalConceptsNeuroscience

room in which the light is always on or always off, flowers often still
undergo a diurnal opening and closing. This is the true test of a circa-
dian rhythm—whether it persists under conditions in which no envi-
ronmental cues to time of day or night are available.
The same is true for animals like us. The periodic variation in
melatonin synthesis by the pineal gland (high at night, low during
the day), of core body temperature, and of sleeping and waking all
continue in the absence of any information about when it is day and
when it is night. This implies that the body contains some sort of in-
ternal timer, an endogenous biological clock, able to track the passage
of time over a twenty-four-hour period.
In addition to circadian biological rhythms, there are other
rhythms of different periodicities. For example, migratory activity in
birds and other animals may be regulated by some kind of circannual
biological clock. Many birds that live in northern latitudes fly south
to warmer climates for the winter and then back north in the spring,
where during the summer they breed and raise their young. One
might suspect that this migratory behavior is triggered by changes in
the temperature or length of the day, things that would clearly mark
the changing of the seasons. Again, this is only part of the story. Stud-
ies have been done with migratory birds born and raised in the labora-
tory—birds that have never been outside. They live their entire lives in
cages within rooms that have no windows to the outside world. They
are also exposed to a constant light-dark cycle: twelve hours of light
and twelve hours of darkness, a cycle that never varies. In addition,
the temperature of the room is kept constant year-round. These birds
get no signals or cues from the environment indicative of the time of
the year and the changing of seasons.
Nonetheless, birds in the northern hemisphere, several months
after their birth, become restless and begin to hop and attempt to fly