Sсiеntifiс Аmеricаn (2019-06)

42 Scientific American, June 2019

O

ne of the most striking features of living organisms,

both animals and plants, is the way their physiology

and behavior have adapted to follow the fluctuations

of daily light and nocturnal darkness. A  clock in the

brain synchronized to environmental cues generates

biological changes that vary over a 24-hour cycle—

circadian rhythms (from the Latin words circa

and diem, meaning “about” and “a day,” respectively). In this way, the earth’s

rotation is reproduced in the dynamics of our neuronal circuits.

The sleep-wakefulness cycle is a typical circadian

rhythm. Wakefulness is characterized by sensory activ-

ity and movement; during sleep the senses lose touch

with their surroundings, and movements subside. This

periodic loss of consciousness appears on electroen-

cephalogram (EEG) recordings as a clear signature:

deep sleep consists of slow oscillations of high ampli-

tude. Wakefulness, in contrast, is made up of fast, low-

amplitude oscillations. Much about sleep remains a

mystery, however. Why would an animal shut down

basic sensory and motor activity for hours on end, leav-

ing itself a target for predators? This question becomes

more acute in aquatic mammals, which need to regu-

late breathing and body temperature while they sleep.

Remarkably, some animals have solved this prob-

lem by developing the ability to sleep with one half

their brain while remaining vigilant with the other—a

behavior known as unihemispheric slow-wave sleep

(USWS). Still others engage in USWS under some cir-

cumstances but put both hemispheres to bed when

necessary. Marine mammals, bird species and possibly

reptiles enter a half-on/half-off state, sometimes keep-

ing one eye open during these intervals. Recently

researchers have even discovered a vestigial form of

unihemispheric sleep in humans.

Half-slumber provides a fascinating vista into the

science of sleep. While studies are carried out on the

dormant half, the opposite side can serve as the requi-

site control for experiments. The ability to thrive with a

relative lack of sleep, as dolphins and some birds do,

Gian Gastone Mascetti is a senior scholar of neurophysiology

in the department of general psychology at the University

of Padova in Italy, where he retired as a professor of neuro­

physiology. His research interests are focused on sleep,

particularly the relation between sleep and brain lateralization.

IN BRIEF

During sleep senses lose touch with their surround-

ings, and movements subside. This interlude raises

the question of why any animal would shut down

basic brain activities for endless hours, leaving itself

a ready target for predators.

Some animals have solved this problem by develop-

ing the ability to sleep with one half of their brain

while remaining vigilant with the other—an in-

between state that goes by the name of unihemi-

spheric slow-wave sleep.

Half-slumber provides a compelling means to study

the science of sleep. Research can be carried out on

the dormant half of the brain, while the other side

be comes a control for experiments. This work may

even help us understand human sleep disorders.