June 2019, ScientificAmerican.com 45the loss of sleep during night flight by in creasing total
sleep time, taking daytime micro naps and, when
perching, closing one eye.WHAT ABOUT US?
humans do not engage in classic USWS, but they occa-
sionally experience something reminiscent of it. Masa-
ko Tamaki and her group at Brown University made
EEG recordings when people spent the night in an
unfamiliar environment. In a 2016 publication by
Tamaki, the EEGs showed slow waves indicative of
deep sleep in the right hemisphere and shallow slow-
wave activity in the left hemisphere, a sign of more
alertness. The left hemisphere, moreover, was more
easily aroused than its opposite half. This asymmetry,
re ferred to as the first-night effect, disappears by the
second night but seems to preserve vigilance in an un -
familiar place. It is reminiscent of mothers who retain
a low awakening threshold to cries or other noises
they identify as coming from their babies.
We may feel the sleep debt after the first night away
from home. But other animals that sleep with one side
of the brain all the time seem to be well adapted to
their routines. Those that immerse themselves in
USWS spend less time sleeping, compared with those
that engage in BSWS or REM sleep.
Even so, their ability to swim, fly, eat or socialize
with companions remains undiminished. Dolphins
spend almost two thirds of their day awake and the
rest of the time in USWS, trading off sleep time
between the two hemispheres. Brain and body recov-
ery, however, does not appear to be affected, de spite
the absence of REM sleep.
In 1997 Mukhametov and his colleagues reported
that dolphins in sleep studies always appeared to be
in good health. In captivity, where scientists could
observe the animals closely, dolphins learned and
memorized complex tasks. Frigate birds sharply
reduced total sleep while flying but maintained a high
level of attention and efficient flight performance dur-
ing their extended journeys.
Some animals seem to cope by sharing the half-
sleep burden. Mallard ducks that act as flock sentinels
by keeping one eye open lose sleep but without
impaired behavior. The birds later pass off their look-
out roles to a companion on another day. Unihemi-
spheric sleep continues to fascinate the research com-
munity because it illustrates the diverse evolutionary
strategies that have emerged to allow animals to get
their rest every day.
The interest generated in USWS from field experi-
ments has even made it into a laboratory tool for
exploring the role of sleep in helping to shape devel-
opment of the brain just after birth. In 1999 my group
in the department of general psychology at the Uni-
versity of Padova in Italy found that just-born chick-
ens ( Gallus gallus ) experienced significantly more
left-hemisphere sleep during the first week after
hatching. The chicks favored that hemisphere in those
early days to learn about stimuli—patterns and col-
ors—that must be processed for the first time by their
new brain: sleep appeared to play a role in organizing
what they had just learned.
Right-hemisphere sleep increased in the chicks as
activity such as spatial analysis and the processing of
new events prevailed on that side in the second week.
When we trained chicks in a color-discrimination task,
they subsequently registered more left USWS (with
their right eye closed and their left hemisphere asleep)
because that hemisphere was dominant in learning
about colors. Chicks used the left eye for a spatial-
learning task that required them to select one of four
containers in a particular corner of their enclosure.
They had to pick the container with a hole on top that
contained a food treat. When they were done, chicks
showed more right USWS (with their left eye closed
and their right hemisphere sleeping) to rest the side of
the brain that specializes in this type of task.
The most active hemisphere—whether en gaged in
USWS or BSWS—spent relatively more time sleeping to
allow for recovery. Meanwhile the open eye on the side
of the nondominant hemisphere took over to watch for
predators and to stay apprised of the environment. In
fact, moving a dark object over the cage during USWS
caused chicks to wake immediately, startle and emit
distress calls. Vigilance remained intact, but it did not
detract from sleep as a time to sort out the intense sen-
sory experiences of the birds’ first days in a new world.
Ultimately studying animals that sleep with half a
brain could aid us in understanding the continuing
biological enigma of sleep—and perhaps even sleep
problems in humans. Apnea and other disorders
sometimes have effects more in one hemisphere than
the other. This work may help answer how a species
balances the benefits of rest with a need to protect
itself against a hungry predator. Sleeping with one
side of the brain is a brilliant answer to this dilemma,
enabling an animal to experience conscious and
unconscious states all at once. Research on unihemi-
spheric sleep resonates through the millennia with a
frequently quoted passage from Heraclitus’ Frag-
ments: “Even a soul submerged in sleep is hard at
work and helps make something of the world.”MORE TO EXPLORE
Cetacean Sleep: An Unusual Form of Mammalian Sleep. Oleg I. Lyamin et al. in Neuroscience &
Biobehavioral Reviews, Vol. 32, No. 8, pages 1451–1484; October 2008.
Physiologically Based Quantitative Modeling of Unihemispheric Sleep. D. J. Kedziora et al. in
Journal of Theoretical Biology, Vol. 314, pages 109–119; December 7, 2012.
Unihemispheric Sleep and Asymmetrical Sleep: Behavioral, Neurophysiological, and Functional
Perspectives. Gian Gastone Mascetti in Nature and Science of Sleep, Vol. 8, pages 221–238; 2016.
Evidence That Birds Sleep in Mid-Flight. Niels C. Rattenborg et al. in Nature Communications, Vol. 7,
Article No. 12468; August 3, 2016.
Sleep in the Northern Fur Seal. Oleg I. Lyamin et al. in Current Opinion in Neurobiology, Vol. 44, pages
144–151; June 2017.
FROM OUR ARCHIVES
Sleeping with Half a Brain. Christof Koch; September 2016.
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