44 Scientific American, June 2019structures within the hypothalamus in each hemi-
sphere—the ventrolateral preoptic nuclei—exchange
messages to regulate when sleep occurs in each hemi-
sphere. It appears that inhibitory signals transmitted
between the two hemispheres could allow one side to
go to sleep while the other stays awake. Deep-brain
structures, such as the posterior commissures in the
brain stem, would also be involved. (The posterior
commissures are extremely large in dolphins, giving
rise to questions about their role in managing sleep.)
The University of Sydney model gives neuroscientists
a way to explore the mechanisms of how the brain
hands off the delicate task of allocating sleep to one
hemisphere or another.
Environmental cues also seem to play a role. Because
the sleep-promoting neurons in the hypothalamus are
thermosensitive, a rise or fall in brain temperature
causes a corresponding fluctuation in the firing rate ofthese neurons. Indeed, in 1982 Mukhametov and his col-
leagues found that during USWS dolphins’ brain tem-
peratures decreased in the sleeping hemisphere and
remained constant in the awake one.A SINGULAR ADAPTATION
cetaceans evolved from a common terrestrial ances-
tor with hippopotamuses and other hoofed mammals.
The move from a terrestrial to an aquatic environ-
ment was gradual and may have included a semi-
aquatic transition that entailed significant physiolog-
ical and behavioral adjustments. Consequently, ceta-
ceans’ sleep behavior represents a singular example of
adaptation to a new environment that demonstrates a
trade-off between the need for sleep and survival.
Other animals make similar compromises. Seals,
for example, have adopted various evolutionary solu-
tions to the closely related problem of breathing and
sleeping in water and on land. Some families of seals
eschew USWS altogether. It has not turned up in ear-
less, or “true,” seals (the family Phocidae), including
harp and elephant seals.
Northern fur seals (the family Otariidae), however,
demonstrate a different story. In 2017 Oleg I. Lyamin of
the A. N. Severtsov Institute of Ecology and Evolutionreported that unlike dolphins, which appear to rarely
experience BSWS and perhaps never enter REM sleep
at all, northern fur seals undergo multiple sleep types,
including BSWS, REM and USWS, in both their aquatic
and terrestrial lairs. On land, BSWS predominates. In
water, the amount of time spent in USWS increases,
compared with that on land. REM sleep in water
diminishes or even disappears.
When immersed in water and experiencing USWS,
fur seals adopt a body posture that allows them to sleep,
breathe and track approaching predators: they lie on
one side with one flipper in the water and paddle con-
tinuously with it while keeping their other three flip-
pers in the air to reduce heat loss. Their nostrils, mean-
while, remain out of the water so the seals can breathe.
The brain hemisphere on the opposite side from the
moving flipper (and the one open eye) is awake, letting
the animals issue motor commands for paddling and
retaining postural stability. On land,
USWS allows fur seals to watch for
predators and coordinate activity with
companions, but it does not help with
breath control, body temperature or
coordination of movement.
Some birds, too, engage in uni-
hemispheric sleep as they balance the
need for rest and defensive alertness.
(At times, USWS is combined with
BSWS and REM.) In 1996 Jadwiga
Szymczak, then at Nicolaus Coperni-
cus University in Poland, recorded the
presence of slow-wave EEGs in one
hemisphere of the European black
bird. And in 2001 Niels C. Rattenborg,
then at the department of life sciences at Indiana
State University, and his colleagues did the same in pi-
geons. Similarly, in 1999 Rattenborg had found that
mallard ducks sleep with only half a brain to watch for
threats. Ducks that kept one eye open while stationed
at the outside edges of a group showed 150 percent
higher levels of USWS than birds located toward the
center. The open eyes of the “sentinel” ducks were di-
rected away from the group. Mark A. Elgar, now at the
University of Melbourne in Australia, reported in a
1989 study that vigilance decreases when the group
grows larger and when an animal moves toward the
center of the group.
Migrating birds during nonstop, long-distance
flights also rely on differing sleep strategies. In 2016
Rattenborg, now at the Max Planck Institute for Orni-
thology in Seewiesen, Germany, and his team studied
USWS and BSWS in great frigate birds ( Fregata
minor ) during their 10-day sojourns. In a single USWS
episode, one hemisphere showed a waking EEG pat-
tern contralateral to the direction of a flight turn,
indicating that the open eye on the opposite side was
watching where the flock was headed. Also, Thomas
Fuchs, then at Bowling Green State University, discov-
ered in 2006 that Swainson’s thrush compensates forWhen immersed in water, northern
fur seals sleep with one brain
hemisphere while adopting
a body posture that allows
them to breathe and stay alert
for approaching predators.