8 Scientific American, October 2019
LETTERS
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DEFENDING MISSILE DEFENSE
Laura Grego and David Wright damning-
ly criticize the U.S.’s Ground-based Mid-
course Defense (GMD) program to inter-
cept incoming nuclear missiles in “Bro-
ken Shield.” But your readers should not
confuse what amounts to a debate over
priorities with a claim grounded in estab-
lished nuclear theory. In fact, ballistic
mis sile defense (BMD) can limit nuclear
damage, buttress U.S. deterrence and em-
power arms control.
The missiles BMD shoots down are
not the only ones that it impacts. Rather
the possibility of directing all BMD to pro -
tect, say, Washington, D.C., forces U.S. ad-
ver saries to allocate additional warheads
to “must kill” targets; accordingly, these
bombs are then unavailable for lower-pri-
ority cities. A missile shield does not have
to be perfect to successfully defend against
an attack from a small nuclear power—
such as North Korea. And BMD raises the
bar for a successful nuclear strike, even for
Russia’s larger nuclear arsenal.
Mark Massa Washington, D.C.
BAR-CODING FIX FOR DRUGS
In “All the World’s Data Could Fit in an
Egg,” James E. Dahlman describes how the
method of DNA tagging could reduce ef-
fort in investigating potential drugs. That
sounds like an awesome development, but
I wonder if the method could be used for
another concern as well.
The fine print for many drugs, particu-
larly psychotropic ones, often says some-
thing on the order of “we don’t really know
how this works,” and they can have severe
adverse reactions, sometimes even in-
creasing the possibility for violence and
suicide. Might DNA bar coding also be
used to reduce such reactions?
Moritz Farbstein St. LouisDAHLMAN REPLIES: DNA bar coding
could eventually alleviate adverse reac-
tions by helping scientists design drugs
that specifically target diseased cells. Many
such reactions occur when a drug meant to
treat a diseased cell also targets a healthy
one. By maximizing the drug delivered to
the site of disease, it may be possible to re-
duce the dose and minimize interaction
between that drug and healthy cells. Using
DNA bar codes, scientists can track how
drugs are delivered to diseased and healthy
cells, all in one experiment. The hope is to
use these data to rationally design drugs
that avoid healthy cells.AWAKE WHILE ASLEEP
“One Eye Open,” by Gian Gastone Ma-
scetti, describes how some animals can
sleep with one half of their brain while
the other stays awake, sometimes keep-
ing one eye open while they do so. The
article particularly caught my interest
because I experienced an effect like this
often while serving on the front lines in
Iraq and Afghanistan.
When I was sleeping, I knew I was
asleep, but I was also awake and aware of
everything going on around me. I was able
to come fully awake in an instant, as if I
had never even been asleep, but I felt rest-
ed. And I could open one eye slightly to
see what was happening around me. This
happened many nights in a row, overmany months. I’ve only experienced it
when my life was in danger, so it was clear-
ly connected to a sense of life and death
hanging on whether I could remain alert.
Mike Scott via e-mailMASCETTI REPLIES: To my knowledge of
human sleep behavior, sleeping with one
eye open seems to be a metaphor or per-
haps a sensation some people experience
when sleeping in new, alarming and po-
tentially dangerous conditions. But I can -
not exclude the possibility that some in-
dividuals might show the capacity to
briefly open one eye and awaken during
sleep as has been reported in some spe-
cies of birds. And we do know that hu-
mans are able to show a sleep strategy
that seems to be reminiscent of the uni-
hemispheric slow-wave sleep in animals
I discuss in my article. When sleeping,
they show a consistent electroencephalo-
graphic slow-wave activity in one hemi-
sphere, indicating a very light sleep level
in the other hemisphere.
A hemispheric sleep asymmetry could
be present in any unusual, unsafe or dan-
gerous environment, allowing the sleeper
to keep a certain level of vigilance. For ex-
ample, it has been shown that sleeping
mothers maintain a high vigilance and a
low awakening threshold to smell, noises
and the cries of their babies. This hemi-
spheric condition should indeed also be
present in soldiers sleeping in a war zone.BAT BUDDIES
In “Deer Friends” [Advances], Joshua
Rapp Learn reports on a study finding that
bats follow white tail deer around to prey
on insects. That observation is not unique:
When my wife is out working on our farm
at dusk, several resident little brown bats
that nest under the shingles of our house
follow her around, preying on mosquitoes.
My wife attracts the bugs, and the bats eat
them—a perfect symbiosis, as the article
notes about the arrangement between bats
and deer. We live almost mosquito-free in
the summer, and our bat friends keep my
wife company at night.
John Davies Sunshine Coast, B.C.ASSISTED MISGIVING
What planet is Wade Roush living on? In
“Safe Words for Our AI Friends” [Ven-June 2019
“Ballistic missile
defense can limit
nuclear damage,
buttress U.S. deter-
rence and empowerarms control.”
Mark Massa Washington, D.C.