THE IMPORTANCE OF SLEEP

 

Sleep is a state that is physiologically and behaviorally distinct from rest or quiet wakefulness. A sleeping animal will immediately rouse with adequate stimulation. This abrupt arousal distinguishes sleep from torpor, hibernation, and other enduring states of inactivity. Most vertebrates do not only engage in quiet wakefulness—they also sleep. The fact that animals must sleep in order to function effectively during wakefulness implies that sleep is biologically important. Apparently, some processes that are essential for homeostatic efficiency and information processing operate exclusively or more effectively in the context of sleep.

Sleep research has largely focused on mammalian sleep and most of these studies used electroencephalography (EEG) to characterize sleep and sleep stages. The problem is that many of the extracranial EEG features of mammalian sleep are not expressed in non-mammalian species. Intracranial recording is highly informative but seldom performed due to technical reasons. Therefore, in reviewing non-mammalian sleep data, behavioral assessment results should take precedence over EEG findings. Behaviorally, sleep is characterized by postural stereotypes, reduction in motor activity, elevation of arousal threshold, arousal following adequate stimulation, circadian periodicity, and homeostatic rebound after sleep loss.

Sleep-wake substates can be delineated in mammals and, to some extent, in birds. The two main types of sleep that were initially identified were named rapid eye movement (REM) sleep and non-REM (NREM) sleep. I prefer to use quiet sleep (QS) instead of NREM sleep and active sleep (AS) instead of REM sleep. In mammals, QS is further divided into light sleep (LS) and deep sleep (DS). QS is often referred to as slow wave sleep (SWS) in animals with no clear LS-DS differentiation. In humans, SWS is synonymous with DS.

Nearly all of the vertebrates and invertebrates studied to date display sleep or sleep-like behavior. This implies that sleep has some adaptive value leading to its selection and evolution. However, sleep should not be viewed as one adaptive trait but as the accumulation of many adaptive traits evolving over time in response to different evolutionary pressures. Some pressures existed in some environments but not in others and some pressures remained robust while others lost their impact with time. Moreover, some animal lineages employed sleep-based strategies to deal with certain pressures whereas others used different mechanisms to adapt to the same pressures. The exposure of animals to variable evolutionary pressures, the evolution of different adaptive strategies, and the fact that sleep is an aggregate of many adaptive traits account for the diversity of sleep phenotypes and sleep-like behaviors in different animal species.

Despite a surge of sleep research in the last few decades only a small percentage of extant animal species have been studied. There is compelling evidence that quiet sleep (QS) and active sleep (AS) are present in homeotherms (mammals and birds). However, evidence suggesting that poikilotherms express AS is either absent (amphibians, fish, and invertebrates) or very weak (reptiles). There is also evidence suggesting that sleep in poikilotherms is not homologous to QS in homeotherms

True, sleep is crucial for overall health and well-being. It plays a vital role in cognitive function, mood regulation, and physical health. Lack of adequate sleep can have negative effects on various aspects of your life.

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