Rodent rhythmicity studies: Use of unfavorable light regimes.
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Animals↗
Biomedical subjects
J Lee Kavanau
Publications and source records attributed to J Lee Kavanau.
Biological time-keeping mechanisms: a need for broader perspectives?
Biological time-keeping mechanisms play fundamental roles in the regulation of behavior and physiology, disruption of which can lead to increased incidence of many disorders. Consequently, these mechanisms continue to be investigated intensively. For almost four decades they have been known to be able to store and program complex behaviors, and to be susceptible to the influences of light and day length. Yet present-day research concerns almost exclusively the means by which circadian and lesser time intervals are measured. Even within this narrow focus disagreements exist. Some early studies of small nocturnal rodents and primates running in exercise wheels illustrate the program-clock-like capabilities of endogenous time-keeping mechanisms and their degrees of susceptibility to external influences, such as artificial twilights and shortened light cycles. Broadening perspectives for research on biological clocks to take into account these often overlooked capabilities and susceptibilities could lead to a deeper understanding of them.
Animals↗
Is sleep's 'supreme mystery' unraveling? An evolutionary analysis of sleep encounters no mystery; nor does life's earliest sleep, recently discovered in jellyfish.
Biotelemetry has revealed daily 15-h behavioral sleep periods in a cubomedusan jellyfish, Chironex fleckeri. Its sleep is expected to be phylogenetically most primitive, since jellyfish possess only two germ layers. They belong to the phylum Cnidaria, the 'simplest' multicellular organisms with an organized nervous system. Cubomedusae have a complex visual system with 24 eyes of four different types, each type specialized for a different task. Input to these eyes during visually guided fast-swimming predation requires enormous amounts of neural processing, possibly nearly saturating the capacity of their comparatively simple nervous system. These heavy neural demands may account for the need for fifteen hours of sleep. C. fleckeri is the only animal known for which sleep may be either present or absent, dependent on lifestyle. Limited knowledge of behavior of some other cubomedusae suggests that they also possess this faculty. The finding of sleep in C. fleckeri supports current proposals of sleep's origin and basic function. Evolutionary analyses link sleep to a conflict produced by excessive processing demands on multifunctional neural circuitry for detailed focal vision by complex lensed eyes. The conflict arises between the enormous demands of complex visual analysis and needs for split-second control of actions, on the one hand, and non-urgent processing of memories of ongoing and stored events, on the other. Conflict is resolved by deferring the non-urgent processing to periods of sleep. Without sleep, selection would favor the evolution of circuitry 'dedicated' to single or but few tasks, with corresponding lesser efficiency. Had complex lensed eyes of medusae originated as a consequence of selection for increased mating success of males pursuing females, it could have occurred before the evolution of fast-swimming bilateral (three-germ-layered) prey. But if it was a consequence of selection for increased prey-hunting success, the origin of such eyes probably awaited the coexistence of bilateral prey.
Animals↗
Evolutionary approaches to understanding sleep.
A major controversy over REM sleep's role in memory processing may owe to inadequate allowances for the highly conservative nature of evolutionary adaptations. The controversy hinges on whether NREM sleep, alone, retains primitive memory processing capabilities. The selective pressure for primitive sleep, is thought to have been the need to obviate conflicts between enormous neural processing requirements of complex visual analysis and split-second control of movements, on the one hand, and memory processing, on the other. The most efficient memory processing during mammalian and avian sleep appears to be a two-step process: synapses in individual component circuits of events are reinforced primarily by slow brain waves during NREM sleep, with the reinforced components temporally bound by fast waves, and manifested as dreams, during REM sleep. This dual action could account for partitioning of sleep periods into multiple NREM-REM cycles. It is proposed that in the absence of REM sleep, all needed memory processing can be accomplished by NREM sleep, alone, though less efficiently. Many symptoms of fatal familial insomnia are attributed to subnormal nightly reinforcement of brain circuitry because of almost total loss of sleep, and compensatory responses thereto during waking. During this disorder, sensory circuitry seemingly is spared by virtue of its supernormal reinforcement during almost continuous waking. Contrariwise, sparing of an adult's 'higher faculties' in encephalitis lethargica appears to owe to supernormal circuit reinforcement during almost continuous sleep.
Awareness↗
Sleep researchers need to bring Darwin on board: elucidating functions of sleep via adaptedness and natural selection.
The development of neural multifunctionality - given brain regions carrying out more than one function - conferred great efficiency on brain function at early stages of evolution. This applied to animals that led relatively simple lives with few needs for long-term memories, such as many lower invertebrates - many molluscs, echinoderms, worms, etc. As more complex lifestyles and detailed focal vision evolved, needs for self-initiated and reflexive activities increased in frequency, and recognition of many locales, conspecifics, and other forms of life became essential. These developments were accompanied by greatly expanded needs for neural processing supporting sensory and motor activities, and establishing and storing long-term memories. Since these categories of neural processing occur in largely overlapping brain regions, brain functioning would have become increasingly maladaptive, had the evolution of these more complex lifestyles not been accompanied by compensating adaptations that obviated these potentially conflicting brain activities. These adaptations consisted of: first, restful waking; second, primitive sleep; and finally, fully developed sleep, with its specialized rapid-eye-movement and non-rapid-eye-movement states, that contribute to the maintenance of great efficiency of brain function. The only animals with detailed focal vision that can achieve highly efficient brain function without sleep, are those in which demands on memory processing are greatly reduced in consequence of routine, monotonous, almost purely reflexive lifestyles, with few needs for acquiring experiential long-term memories. The best known animals in this non-sleeping category are tunas and many sharks.
Adaptation, Physiological↗
REM and NREM sleep as natural accompaniments of the evolution of warm-bloodedness.
Divergence of primitive sleep into REM and NREM states is thought to have occurred in the nocturnal Triassic ancestors of mammals as a natural accompaniment of the evolution of warm-bloodedness. As ambient temperatures during twilight portions of primitive sleep traversed these evolving ancestors' core temperature, mechanisms of thermoregulatory control that employ muscle contractions became superfluous. The resulting loss of need for such contractions during twilight sleep led to muscle atonia. With muscle tone absent, selection favored the persistence of the fast waves of nocturnal activity during twilight sleep. Stimulations by these waves reinforce motor circuits at the increasing temperatures of evolving warm-bloodedness without leading to sleep-disturbing muscle contractions. By these and related interlinked adaptations, twilight sleep evolved into REM sleep. The daytime period of sleep became NREM sleep. The evolution of NREM and REM sleep following this scenario has implications for sleep's maintenance processes for long-term memories. During NREM sleep, there is an unsynchronized, uncoordinated stimulation and reinforcement of individual distributed component circuits of consolidated memories by slow wave potentials, a process termed 'uncoordinated reinforcement'. The corresponding process during REM sleep is the coordinated stimulation and reinforcement of these circuits by fast wave potentials. This action temporally binds the individual component circuit outputs into fully formed memories, a process termed 'coordinated reinforcement'. Sequential uncoordinated and coordinated reinforcement, that is, NREM followed by REM sleep, emerges as the most effective mechanism of long-term memory maintenance in vertebrates. With the evolution of this two-stage mechanism of long-term memory maintenance, it became adaptive to partition sleep into several NREM-REM cycles, thereby achieving a more lengthy application of the cooperative sequential actions.
Adaptation, Physiological↗
Dream contents and failing memories.
Mentation during sleep states is thought to originate in an activation of brain circuits that encode inherited and experiential memories. Spontaneous degradation of the strengths of synapses occurs in all brain circuits because of "turnover" of molecules essential for synaptic function. In circuits employed frequently during waking, synaptic strengths are refreshed and maintained in their dedicated or functional ranges largely through use, by virtue of activity-dependent synaptic plasticity. In circuits employed infrequently during waking, synaptic strengths are refreshed largely during sleep, by circuit activations induced by spontaneous, self-generated, largely low-frequency brain waves, also by virtue of activity-dependent synaptic plasticity. The outputs of circuits activated during sleep do not necessarily rise to the level of 'unconscious' awareness. Such an absence of awareness of the outputs of individual circuits, that is, an absence of dreaming, is proposed to be the primitive condition in animals that sleep. On the other hand, temporal binding of these outputs is accompanied by the thoughts and perceptions of dreams, which is proposed to be the advanced condition. Linking or serial ordering of otherwise 'static' thoughts and perceptions gives rise to continuous, often narrative and veridical, dreams. In all cases, dream contents are derived from the memories--not necessarily veridical--encoded in the reinforced circuitry. In the absence of synaptic strength refreshments during sleep, synaptic strengths in infrequently used circuits would weaken and the circuits would become incompetent, with their encoded memories degraded or lost. Maintenance of synaptic strengths in infrequently used circuitry during sleep apparently does not always achieve perfection. Weakened synapses begin to occur in circuits in appreciable numbers in children after the age of about 5 years. When these 'incompetent' circuits (with weakened synapses) are activated during sleep, there are minimal influences on dream contents, namely, distortions that make some objects, such as animals, faces, and scenes, unrecognizable. As weakened synapses increase in numbers with age, the numbers of distorted objects in dreams also increase. In adults, people in as many as 80% of dreams may be unrecognizable. Besides the normal weakening of synaptic strengths, some synapses become defective, in consequence of deleterious, adventitious, exogenous influences, for example, radiation. As these faulty synapses accumulate in old memories, activation of circuits incorporating them during sleep leads to dreams with incoherent, bizarre, or impossible contents. The infrequent activation of such old, incompetent circuits in some waking conditions leads to false memories, delusions, or hallucinations.
Aging↗