| MadSci Network: Neuroscience |
From what I read our readiness to sleep seems to have at least 2 peaks (see
image 1) and follows a circadian rhythm, i.e. these peaks usually
(with
minor variations) appear at the same time of the day, even under conditions
of sleep deprivation (see figure 9 on
the UCLA sleep tutorial - you need to scroll down the page a little).
Researchers found this out when they observed test persons either under
normal conditions or under conditions where they had eliminated all
external zeitgebers (cues such as the day-night cycle) in so called
isolation or bunker experiments. Recording the test persons' behavior, such
as sleeping and waking, while simultaneously monitoring several
physiological parameters, showed that peaks in sleepiness were in the
majority of cases strongly correlated with a minimum of (core) body
temperature in the free-running conditions (without external zeitgebers)
and occured with a circadian period of appr. 25 hours. In addition,
adrenaline levels (as determined from urine samples) also reached their low
then. (Adrenaline is usually associated with an aroused/alert state.) In
addition, several other parameters, such as blood pressure, heart rate,
etc.followed suit as well.
Under conditions where we have external cues humans reset/synchronize their
internal clock with the environment every day and then one observes a more
or less constant low at around 4 or 5 a.m. in most persons. See figure 5
here.
Afternoon napping undisputedly occurs but hasn't been a research focus up
until about 10 or 15 years ago. In fact, in a lot of laboratory settings
(bunker experiments) napping was even actively discouraged (although test
persons found ways and means to circumvent this 'prohibition'). Experiments
revealed that when given the choice most test persons included a nap into
their sleep schedule.
In the words of Jürgen Zulley, a researcher in the field of
sleep pathology, naps constitute a second preferred timepoint where sleep
can, but doesn't have to, occur. Moreover, '...[although] napping habits
vary across cultures, research shows that napping is a common feature of
healthy adult sleep-wake behavior, even when no sleep debt was incurred the
night before the nap.'
(quoted from http://www.cb
t.virginia.edu/tutorial/SLEEPWAKE2.html).
Naps can nevertheless be suppressed or avoided (in experimental terminology
this is called masking) without any physical harm, i.e. they do not seem
instrumental in physical recuperation (unlike night time sleep deprivation
which will cause severe psychological effects such as an inability to
concentrate, moodiness, delusions, etc. in humans and can be lethal in
rats).
As to the evolutionary relevance of naps I haven't been able to come up
with a totally convincing explanation. As mentioned before, afternoon
naps occur when there is a temporary trough in some physiological
parameters, among them most notably body temperature, pulse rate, the
ability to concentrate, etc. all associated with decreased physical and
cognitive activity.
Sleep has some restorative function (as can be told from sleep
deprivation experiments) but whether it pertains to physical
or cognitive functioning is still a matter of intense research. For some
interesting theories, refer to this source.
(For some odd reason the internal linking on this Web site doesn't work
correctly, so you'll have to click to Chapter I yourself - sorry!).
In addotion, note that some bacterial cell wall components can elicit
sleeping (you usually sleep a lot when you're sick). This might point to a
role in the immune system's proper functioning.
It seems as if humans weren't built for prolonged work periods although can
with some effort override this intrinsic desire for rest. Interestingly, a
great number of traffic and occupational accidents occur at exactly those
times when we hit a low. Naps of 10 to 30 minutes (no longer, though!) are
actually nowadays being recommended by some occupational physicians (but of
course they're not exactly strongly encouraged in Western society; that's
somewhat different in some Asian and Southern European countries (siesta
time!) and in Africa.).
For further request, contact me at this
email address.
References:
Try the links in the MadSci Library for more information on Neuroscience.