Unihemispheric Sleep: An Enigma for Current Models of Sleep-Wake Regulation.

Some mammals (whales, dolphins, fur seals, sea lions) sleep with one hemisphere of the brain being asleep while the other is awake.1,2 This is referred to as unihemispheric slow wave sleep (USWS) and contrasts with the bihemispheric slowwave sleep (BSWS) exhibited by humans and other mammals. Whales (Delphinapterus leucas) and dolphins (Tursiops truncates) show only USWS.3,4 Northern fur seals and sea lions (family Otariidae) are aquatic and terrestrial. While in water these animals have USWS, like cetaceans, but on land they have both USWS and BSWS.3 It is unclear whether cetaceans have REM sleep, whereas Otariidae have REM sleep on land, and it is always bilateral. Some birds also have USWS,5,6 but neurochemicals related to USWS have only been measured in the fur seal. The evolutionary basis of USWS is unclear.7,8 Polysomnography studies have conclusively shown that USWS is indeed sleep because one hemisphere has high-amplitude slow wave activity (1.2–4 Hz), while the other hemisphere has desynchronized EEG activity.5,6,9 The daily quota of sleep is equally distributed between the hemispheres.10 If the hemisphere with USWS is repeatedly interrupted, then that hemisphere will have rebound USWS indicating a homeostatic need for USWS in that hemisphere.5,6,9 In such unihemispheric sleep deprivation studies, there is no compensatory increase in sleep in the non-deprived hemisphere, indicating that the homeostatic need for sleep accumulates independently in each hemisphere. These data indicate that USWS is actively generated and there is a compensatory need for it. The discovery of unihemispheric sleep is a boon to sleep research as it provides a unique opportunity to empirically test neural circuit models of sleep-wake regulation. Lyamin, Mukhametov, Siegel and colleagues have clearly recognized this and have conducted elegant studies in northern fur seals (Callorthinus ursinus) to test the hypothesis that neurochemicals linked to wakefulness are elevated in the awake hemisphere compared to the hemisphere with USWS.11–13 In these studies the seals were instrumented to record the EEG, EMG, and EOG, and also implanted with pairs of guide cannulas targeted in symmetrical positions in each hemisphere. Microdialysis probes collected extracellular fluid from each site every 10 minutes and the levels of acetylcholine, histamine, norepinephrine (NE), and serotonin in the sample were assessed. The seals were maintained on a 12–12h light-dark cycle and samples were collected in active waking, quiet waking, BSWS, USWS, and REM sleep. Acetylcholine was measured in the first study,12 serotonin in the second study,11 and in a study published in the current issue of SLEEP, Lyamin and colleagues13 measured levels of histamine, norepinephrine, and serotonin. pii: sp-00020-16 ht tp://dx.doi.org/10.5665/sleep.5508