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As this quote by Aristotle
illustrates, the question of whether insects and other lower animals
sleep was postulated well before the advent of the Common Era. Interestingly enough, however, although
contemporary scientists have confirmed a sleeping behavior in most classified
insects, we do not seem to be very much closer than the ancient Greeks
were to understanding the nature of sleep.
As our knowledge and mastery of the sciences has progressed immensely
over the past two millennia, giving us new tools with which to study
sleep and sleepers, the intricacies of the nature of sleep continue
to be revealed. Along with every sleep mechanism that
she allows to be uncovered, Mother Nature also gives a knowing wink,
no doubt leading every student of sleep behavior to wish -- at least
once --that Aristotle's definition would suffice, settling the dispute
once and for all. Although some of Aristotle's specific
observations regarding the phylogeny of sleep may seem outdated, they
have served as a springboard for countless new discoveries. Over the years, testing for sleep in insects
has evolved from a simple yes/no investigation to complex metabolic
and genetic tests that may yet reveal links to human physiology and
revolutionize sleep science. The
mystery of sleep in insects is still not yet fully understood and it
is a relief to realize that Aristotle did not ruin the surprise for
us after all.
Traditionally, four types of behavioral criteria have been used to verify
the presence of sleep. These include minimal movement, a typical sleep
posture (e.g., for humans, lying down; for bats, hanging upside down),
reduced responsiveness to external stimulation, and quick reversibility
of reduced responsiveness with relatively intense stimulation (in contrast
to death, anesthesia, and coma). 1 It must be noted, however, that these
characteristics should not necessarily be expected to accurately represent
the sleeping habits of all members of the animal kingdom. Since various
animal species differ so drastically in their anatomy, physiology, and
modes of adaptation to their environment, it would be remarkable indeed
if one set of sleeping habits were attributable to all. 1
To date, sleep studies have predominately focused on mammals.
The first scientifically valid investigation of sleeping habits
of invertebrates was conducted only twenty years ago. This study looked
at the effects of sleep deprivation in cockroaches. There are only one million
known invertebrate species out of an estimated existing ten to fifty
million, therefore no simple generalization on the basis of a few observations
can account for the considerable diversity that must exist. 2 Some insects, especially aquatic ones
and those that live near the poles, may have different daily cycles,
or even maintain round-the-clock activity. 2 For this reason, the term 'torpor' seems to provide a more
inclusive and accurate term for describing sleep activity in insects.
An insect in torpor exhibits immobility and distinctly reduced response
to stimuli, although it can rouse from torpor in a matter of seconds
given a strong enough stimulus. Even among insects that do exhibit torpor,
the degree and nature of its expression are somewhat variable. 2
One
of the more dramatic forms of torpor is seen in some bees (chiefly in
the family Apidae),
which firmly clamp onto a plant with their jaws in the evening. 2 They then fold up their legs and maintain
this odd, dangling pose all night long, until they rouse the following
morning. The species that
observe this ritual use the same spot every evening, presumably marking
it with some chemical that they can detect from a distance. 2 This very specific behavior is regarded
by many researchers as the closest thing any insect has to conventional
sleep. 2
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Instead of gauging the
degree to which insects sleep using the behavioral criteria checklist,
scientists have devised clever ways of establishing an experimental
checklist to test for sleep in insects. Since many insects are too small
to have their brainwaves measured, scientists have implemented metabolic
and genetic tests for sleep. For
invertebrates, elements such as body position, arousal threshold, muscle
activity, and heart rate have been identified, which allow researchers
to differentiate rest from sleep. 3
The most celebrated results
from insect sleep studies have stemmed from Drosophila experimentation.
In Drosophila,
researchers tested for sleep by videotaping the flies during rest periods
to document the insects behavior.
Each night, the flies crawled off to resting places and settled
into what the researchers defined as a sleep pose, slumped 'face down'.
4 For about seven hours every night, the
flies stayed still except for occasional twitches of the legs and proboscis.
4 As the evening progressed, louder and
louder taps on the cages were needed to rouse the insects. In some sessions, scientists tapped on
the containers to keep the flies from resting. The sleep-deprived insects compensated by increasing the amount
that they slept over the next few days, as sleep-deprived people do.4
Since
some researchers have speculated that a buildup of adenosine in the
brain may cause sleepiness in mammals, a test was conducted in the flies
to see if a similar response would occur. 4
One team of researchers had the flies consume caffeine, which
disrupts adenosine action in mammalian brains.
Interestingly, the flies indeed slept less after ingesting caffeine
and rested more after eating an adenosine-mimicking compound. 4
Further
genetic testing in Drosophila has
involved the disruption of two genes described as period and timeless
genes, respectively. 4 In flies without the period gene,
no sleep behavior abnormalities were noted but the timeless-mutant flies
did not seem to catch up on sleep after being rest-deprived. 4
image courtesy http://bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-06/Tardigrade_1/tardigrade_1.htm |
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As the methods by which sleep testing in insects improves,
some procedures may be developed that may prove applicable to humans,
presenting an insect world of potential for furthering our understanding
of sleep as well as providing possible means of combating sleep disorders.
Expanding their research from genetic investigation of sleep
in fruit flies, scientists can now conduct genetic experiments with
more complicated animals such as mice. As Drosophila geneticist Jeff Hall concludes, 'Just as the little
fruit fly cracked the biological clock, exposing many of its mechanisms,
[insects] will eventually reveal more genetic links between sleep and
the biological clock'. 4
References
Movie courtesy http://www.microscopy-uk.org.uk/mag/artmay99/dwbear.html.
