Losing Sleep
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In addition to helping physicians diagnose disorders, stable and unstable patterns reveal something about sleep quality that doesn’t involve measuring age-variable slow-wave sleep. While healthy 60-year-olds show little slow-wave sleep on an EEG—apparently, older people get much less SWS—their ECG-derived spectrograms show plenty of stable sleep. Indeed, Thomas thinks that stable patterns, which aren’t captured at all in standard sleep studies, could be an effective measure of sleep quality. In stable sleep, the brain, the heart and respiration become calm, whereas in unstable sleep, everything fluctuates as on a choppy sea—the brain undergoes more frequent microarousals, or CAP, and heart and respiration rates rise and fall, for instance—and this produces a less restorative slumber.
Because neither CAP nor stable-unstable sleep patterns correspond with standard sleep stages, many sleep physicians are skeptical of their importance. But others, including Verma, welcome this research as an attempt to explain what’s going on with patients who don’t meet the criteria for sleep apnea but whose sleep isn’t refreshing. And Virend Somers, a consultant cardiologist at the Mayo Clinic in Rochester, Minn., likes having an alternative approach for evaluating the quality and nature of sleep and identifying its pathologies. “What happens to the heart during sleep, as measured by an ECG, is an important emerging area,” Somers says.
Yet Verma doesn’t think the sleep spectrograms will soon replace the EEG as a diagnostic tool. Despite the statistical association between CAP and ECG-measured unstable sleep, directly gauging CAP activity on an EEG is much further along, in terms of scientific validity and widespread acceptance, than the new ECG-based approach, Verma says. Still, the ECG research, plus several other independent lines of inquiry into alternating brain patterns, has finally begun to modify our 40-year-old conception of what sleep is. As Sejnowski says of his powerful new model for recognizing previously invisible EEG patterns, “We’re zooming in on the nuances of sleep and finding things no one has ever seen.” It could take years before the average sleep clinic is equipped to uncover such nuances. Racing against the almost-daily news about the ill effects of poor sleep, it can catch up none too soon.
