If a tree doesn't fall in a forest . . . ?

摘要

Sleep is a risky business. It involves immobility, lowered metabolism, unconsciousness and dramatically elevated predation risk. That sleep is a ubiquitous behavior in highly developed animals, despite its inherent risks, indicates its importance and evolutionary adaptive value. Put simply, humans, and most animals, need to sleep; yet they also need to wake up if something happens in the environment that signals elevated risk of danger. Fortunately, one important characteristic of sleep, even deep sleep, that makes it different from other forms of unconsciousness (e.g. coma), is that it is rapidly reversible. Arousal thresholds underlying this reversal vary as a function of sleep state, circadian timing, prior sleep debt and many other factors. Clearly, the ability to arouse from sleep requires careful calibration. If the arousal threshold is too high, injury or death could occur. For example, even extremely sleep deprived patients with severe obstructive sleep apnea syndrome will awaken briefly to breathe, although the stimulus required to awaken them (in this case intrathoracic pressure) is much higher than in non-affected individuals. Conversely, if the arousal threshold is too low, sleep will be disrupted and its restorative value diminished. This is the case with some insomnia patients, who will awaken to sounds that most people would comfortably sleep through. The paper in the present issue of Clinical Neurophysiology by Muller-Gass and Campbell (2014) addresses an important question related to how the balance between appropriate awakening and sleep maintenance is regulated; that is, what is the sensitivity of the sleeping nervous system to changes in the external environment.