Mice don’t own alarm clocks – so how do they
know when it’s time to wake up? Mice are nocturnal
which means they are most active at night. Something
inside of them causes them to wake up in the evening
and go about their business. This behavior is probably
useful since the darkness hides them from predators.
Similarly, owls do not get up with the sunrise, but
rather hunt at night when some unlucky mouse is likely
to become their dinner. It’s clear that the ability of
these creatures to match up their behavior with nature’s
rhythms (like the day and night cycle) is critical for
their survival.
The activity patterns of mice and owls are
examples of biological rhythms that are synchronized
to the day/night cycle. These patterns are called
circadian rhythms. This phrase has Latin roots. Circa
means “about” and dian means “a day.” A circadian
rhythm cycles over a one day period.
Other biological rhythms are tied to the tides or the
season which are much longer (from hours to months).
Circadian rhythms are important because they
influence so many aspects of our daily lives. These
rhythms influence everything from when we get tired,
to when we feel at our best, to when we are hungry.
Scientists have discovered that our bodies have
over 100 biological rhythms. For example, our blood
pressure peaks at 7 a.m. and our different hormone
levels rise and fall throughout the day. These
discoveries can affect everything from when we should
take certain medications to safety precautions for
people working night shifts.
As scientists began studying circadian rhythms,
they asked questions such as, “How are these rhythms
maintained?” and “What keeps them so regular?” One
idea was that something in the environment – outside
of the organism –kept the rhythm going. For example,
some scientists hypothesized that the rising or setting
of the sun must control the wake/sleep cycle in
animals. This would mean that the owl or mouse would
have to somehow sense the setting of the sun in order
to become active.
Other scientists argued that there had to be more to
it than that. For example, it is often not enough for an
animal to just react to some change in its environment
(like the sun coming up). Sometimes they have to
anticipate and be ready for such events in order to survive.
These rival hypotheses demonstrate how
scientists can have differing ideas to explain the same
natural phenomenon.
Consider the example of the earthworm. Earthworms
come out at night and retreat underground before the sun
comes up. If the worm is going to avoid being seen by
predators or harmed by the sun, it has to retreat
underground before it starts getting light. In other words,
if the worm wants to avoid the early bird, it has to
anticipate the rising of the sun. This simple example
suggests that something more than external cues are used
by organisms to set their daily cycles. This argument says
that circadian clocks offer organisms a survival benefit by
allowing them to anticipate events such as sunrise and
sunset.
In order to figure out whether or not external factors
control circadian rhythms, scientists have performed what
are called “free-running” experiments. In these
experiments, the scientists create a controlled
environment where there is constant darkness. If you were
inside one of these environments, there would be no way
to tell if it was day or night.
Scientists have used free-running experiments to
study sleep cycles in many organisms, including humans.
In these studies, people live in a controlled environment
where there are no clocks and they can’t see outside to
know whether it is night or day. Remember, in order to
test the truthfulness of a scientific explanation, it’s not
enough to JUST have an idea or hunch, but scientists
must conduct carefully controlled experiments.
In one experiment, a person spent almost two months
living in a deep cave where he was unable to tell if it was
night or day outside. He carefully recorded his sleep/wake
cycle the whole time he was in the cave. This record of
his sleeping and waking was the scientific data (evidence)
that have helped scientist discover some interesting things
about human sleep cycles (explanations).
In a little while you will look at the actual data from
the cave experiment, but first you need to understand a
few things about how scientists measure circadian
rhythms. To better understand how scientists measure and
interpret sleep cycle data, you are going to examine data
from a free-running experiment with a human being.
