Unihemispheric Slow-Wave Sleep

What is sleep?

Sleep is one of the most prominent animal behaviors. Humans spend 1/3 of their lives in this behavioral state, and many mammals spend even more (13). The definition of sleep may seem obvious; behaviorally, sleep is a period of rest in a species-specific posture. Further, increased arousal threshold and rapid transition to the waking state characterize sleep behavior (23, 26). Sleep can also be defined physiologically; in this sense, sleep has characteristic eye movements, muscle tonation, and cortical rhythms as monitored by electroencephalography (EEG).

This technique uses multiple electrodes, placed either on the scalp, or (if ethics and experimental setup allow), directly in the cerebral cortex. The voltage potential measured at one electrode is subtracted from the others, returning a measurement in the µV to mV range. The potentials these electrodes record are measures of the summation of post-synaptic potentials in many cells covering a (relatively) large area of the brain. In this way, EEG is able to record, with sub-millisecond temporal resolution, gross changes in electrical states of areas of the brain.

Using EEG, characteristic oscillatory patterns in cortical activity have been described and used to define the stages of mammalian and avian sleep. Other parts of the brain may also be producing correlated wave patterns (5), but the oscillations of the cortex are much better characterized becuase of their accessibility to EEG technology. The waking state of an organism is characterized by low amplitude, high frequency cortical waves (Figure 3) known as alpha rhythms (8-12 Hz) (30) which appear like this on an EEG:

Figure 1. Alpha waves as they appear in EEG of an awake human with closed eyes. From: reference 34.


In mammalian and avian sleep, there are two broad categories of cortical activity: NREM and REM. The EEG of REM sleep is similar to that of a waking animal (Figure 3), while during NREM sleep the EEG is composed of high amplitude, low frequency waves characterized by slow delta (<4 Hz) and theta (4-8 Hz) cortical waves (Figure 2); because of this, NREM sleep is also called slow-wave sleep (SWS) in birds and non-human mammals.

Figure 2. Delta and theta EEG activity is shown in the red boxed area, as recorded during slow-wave sleep. Image is public domain.


REM sleep is defined by a marked transition from the delta and theta waves of SWS to higher frequency alpha and beta (>12 Hz) waves as well as desynchronized patterns (30).

Other sleep states, such as the sleep-spindles, have been observed; these high frequency bursts (12-16 Hz) occur during mammalian SWS, but have yet to be seen in avian sleep (23).

Figure 3. Representative electroencephalography (EEGs) from a laboratory mouse while in a) slow-wave sleep (SWS) b) REM sleep and c) awake. Plots to the right indicate the relative power (in arbitrary units) of various wave frequencies in the adjacent EEG. From reference 13.


What is unihemipsheric slow-wave sleep?

Some birds and higher-order marine animals display unihemsipheric slow-wave sleep. Behaviorally, this is characterized by sleeping with one eye open and one eye closed; electrophysiologically, half of the cerebral cortex displays the characteristic oscillation and rhythms of sleep while the other half the characteristic neural patterns of the waking state (Figure 4).

Unihemispheric sleep lacks a REM state, displaying only SWS oscillations (23).The division between the sleeping and non-sleeping halves of the brain is not sharp; it is to some extent unclear if the physiological state of the unihemispherically sleeping brain corresponds directly to one of the states of the classic bihemispheric sleep/wake cycle (24).


Figure 4. EEG recording from the left and right hemisphere of a bottlenose dolphin parieto-occipital cortex (A) during unihemispheric slow-wave sleep with the left (B) or right (C) hemisphere asleep. From reference 23.