News Release

A new animal study clarifies how different parts of the brain regulate sleep

Peer-Reviewed Publication

American Physiological Society

(January 21, 2003) - Bethesda, MD – New studies have revealed that even the best-behaved and careful drivers may encounter a new danger on the road – themselves. According to the British journal Occupational and Environmental Medicine, researchers in Australia and New Zealand report that sleep deprivation can have some of the same hazardous effects as being drunk. They found that people who drive after being awake for 17 to 19 hours performed worse than those with a blood alcohol level of .05 percent, the legal limit for drunk driving in most western European countries, and close to the limit of .08 in several American states.

Background

In the United States, the National Sleep Foundation and sleep experts believe that there are other problems associated with sleep deprivation in addition to problems behind the wheel. Those with too little sleep may have higher levels of stress, anxiety and depression, and may take unnecessary risks. Past research has identified the cause for sleep deprivation to be obstructive sleep apnea, resulting from failure of the body to obtain adequate respiration during sleep.

Others believe that brain activity could be the key to sleep deprivation. The electroencephalogram (EEG) has been found to effectively investigate sleep regulation in both humans and animals. In particular, EEG slow-wave activity (SWA, EEG power between 0.75 and 4.0 Hz) is a reliable indicator of non-rapid eye movement (NREM) sleep intensity. It is determined by the amount of prior waking and sleep, and may serve as a marker of sleep homeostasis. The enhancement of SWA by prolonged waking and its subsequent monotonic decline during sleep was documented for humans and several animal species. These observations suggested that SWA could be a recovery process occurring during sleep.

EEG power within the delta band showed a frontal predominance in human NREM sleep, a feature that was most prominent in the first part of the night and was enhanced by sleep deprivation (SD). In rodents, a six hour sleep deprivation period gave rise to a larger SWA rebound in the frontal derivation than in the occipital derivation. Spindle frequency activity (SFA; power in the 12- to 15-Hz band in NREM sleep), another marker of human sleep regulation, also exhibited a specific topographic pattern. After prolonged waking in humans, it showed the largest decrease in the centro-parietal region. In the rat, prolonged waking reduced EEG power in the spindle frequency range (10.25–16.0 Hz) more in the frontal derivation than in the occipital derivation.

In addition to the regional differences along the anteroposterior axis, interhemispheric asymmetries in the sleep EEG may be of functional significance. Spectacular examples are dolphins which exhibit "deep" slow wave sleep only in one hemisphere at a time.

State-related interhemispheric EEG asymmetries favoring the right hemisphere in NREM sleep and the left hemisphere in REM sleep have been reported for humans, cats, and rabbits. In a recent human study, power in the 4- to 8-Hz band of the centro-parietal derivation was higher on the right side in NREM sleep and on the left side in REM sleep. Power within the spindle frequency range (11–15 Hz) in NREM sleep exhibited a left-hemispheric predominance.

For the first time, researchers have unearthed evidence for an opposite EEG asymmetry in high-intensity NREM sleep and REM sleep in the rat. With some state-related interhemispheric variations of the EEG reported for humans, these findings provide further support for the notion of use-dependent local sleep. The anterior predominance of low-frequency activity in the NREM sleep EEG during high sleep pressure may reflect the increased vulnerability of the frontal cortex of the brain to a sleep deficit and its higher need for recovery during sleep. The left predominance at the parietal cortex may be due to a preferential unilateral sensori-motor activation during spontaneous waking. It is known that there is a higher occurrence of right "handedness" in rodents. In addition, the state-related alternation of hemispheric dominance between the major EEG markers of NREM sleep and REM sleep could indicate that the NREM-REM sleep cycle modulates the functional relations between hemispheres.

The Study

The authors of "Interhemispheric Sleep EEG Asymmetry in the Rat is Enhanced by Sleep Deprivation," are Vladyslav V. Vyazovskiy, Alexander A. Borbely, and Irene Tobler, all from the Institute of Pharmacology and Toxicology, University of Zurich, Zurich, Switzerland. Their findings have been published in the November 2002 edition of the Journal of Neurophysiology, one of 14 journals published each month by the American Physiological Society (APS).

Methodology

To investigate their possible functional significance, the cortical EEG of the rat was recorded from frontal and parietal derivations in both hemispheres. Records were obtained for a 24-hour baseline day, six hour sleep deprivation (SD), and a subsequent 18 hour recovery.

Results

The low-frequency range of the NREM sleep EEG is of particular interest because it is a reliable marker of homeostatic sleep regulation and an index of sleep intensity. These results agree with previous observations in the rat and mouse that a subdivision of slow-wave activity into a higher and a lower frequency band revealed further regional differences in the animals' time course during recovery sleep. This heterogeneity in the low frequency range was evident in the present study also on a regional scale where a low-frequency component (around two Hz) was less affected by sleep deprivation.

The present study confirmed that low-frequency power declines progressively during the light period, the rat's main-sleep period. On the other hand, there is an enhancement of low-frequency power during sleep deprivation. The novel aspect of the present study is the demonstration that these changes do not occur uniformly over the entire cortex but differ along the antero-posterior and left-right axes. The results are relevant for the tenet of "local sleep", which postulates that in addition to being a global brain phenomenon, sleep has also a local aspect.

Regional differences in the sleep EEG are assumed to reflect different intensities of local sleep.

The frontal predominance of the baseline EEG at the beginning of the light period was enhanced further under increased sleep pressure. Thus the increase in low-frequency power in the NREM sleep EEG encountered after sleep deprivation exhibited a prominent antero-posterior gradient. The increase in power was larger at the frontal derivation than at the parietal derivation.

Conclusions

This study offers a significant contribution to understanding how the brain, and the factors leading to dangerous deprivation regulate sleep.

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Source: November 2002 edition of the Journal of Neurophysiology.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.


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