Exploring Phylogeny to Find the Function of Sleep
The phylogenetic framework depends on the postulation that sleep states emerged early in animal evolution
Certain functional aspects are probably conserved from an ancestral sleep state
Sleep evolved fundamentally as a metabolic state of the animal and serves functions outside the nervous system
Sleep serves metabolic roles in humans
Some animals sleep more deeply, whereas others extend the duration of their sleep or sleep at inappropriate times
Behavioural response to sleep deprivation provides evidence of sleep’s importance
Sleep during lethargus has been termed developmentally timed sleep
Sleep can be found in an animal that has a mere 302 neurons and that lacks overt circadian behavioural rhythms
Jellyfish show sleep behaviour, demonstrating that neither a brain nor a CNS is required for sleep
Cockroach: when forced to continue locomotion during its resting time, showed an increase in subsequent immobility → suggests homeostatic guarding of the resting behavioural state
Honeybee: reduced arousability was associated with decreased responsiveness of visual interneurons, demonstrating a physiological basis to this behaviour
Fruit flies: prolonged immobility during the night, are less responsive to stimulation during their quiescent period and, following forced movement during the sleep period, display immobility during the early day
Have a clearly defined CNS with a brain with an approximately 24h circadian periodicity
Jellyfish: the frequency of their pulsations is higher during the day than during the night, their responsiveness is reduced at night, and forced movement at night results in reduced activity and reduced responsiveness during the subsequent day
There is more evidence supporting that sleep existed in a common ancestor than that sleep evolved convergently
Evolutionarily distance species share common sleep functions
Claims of a wholly sleepless animal must be viewed with scepticism
REM and NREM sleep occur in mammals and birds and may also be present in reptiles, suggesting that among vertebrates it was present at least as early as when amniotes evolved more than 300 million years ago
Brain neurons are top-down regulators of sleep–wake states in mammals
There is evidence that there is a bottom-up organization
As electrical activity of neurons within local neural groups becomes synchronized with the activity of neurons in other local neural groups, behavioural sleep emerges at the organismal level
The local sleep-like state during wake might interfere with the role of the network in normal wake behaviour and is not as effective as organismal sleep at achieving global sleep functions
When local neural groups eventually become coordinated, the organism shows sleep behaviour
Sleep and wake lie on a continuum
Neither central regulatory neurons nor an intact organism are needed to generate sleep states
The influence of signals from non-neuronal tissues on sleep regulation suggests that the ancestral function of sleep might have resided in non-neuronal cells
Sleep and wake might be controlled by endocrine signals
Non-neuronal cells producing endocrine signals might also be able to influence motor behaviour and reduce sensitivity to external stimuli, resulting in sleep behaviour
There is sleep in animals that lack neurons
If, at its core, sleep were serving a metabolic function, it is not inconceivable that plants, algae and single-cell prokaryotes will also ultimately be considered to sleep
In humans and other terrestrial mammals that are born in an underdeveloped state, sleep need is greatest during development
Increased sleep is apparently coupled to somatic and neural growth and development
Mammals, arthropods and nematodes also all sleep more in the setting of either an infectious or non-infectious illness
Metabolic temporal compartmentalization may serve not only to separate chemically incompatible reactions but also to divert energetic resources used in neural processing to other uses, whether in the brain or in the body
Overall metabolic costs may be reduced by restricting such processes to run at high capacity for a limited portion of the day
The more waste that has been accumulated, the more that can and will be cleared
Emergence of numerous tissue-specific and organism-specific molecular changes during both sleep and wake
Sleep is a time for anabolic activity and the building of macromolecules in all cell types
Within the brain, different regions and cell types show unique patterns of sleep-modulated transcription
Sleep-induced savings in neuronal energy and anabolic metabolism may be conducive to neural plasticity
Sleep emerged early in animal evolution
Metabolic demands are a key driver of sleep regulation
Sleep itself serves primarily metabolic roles
Once the essential function of sleep is understood, it may come to be appreciated in organisms that lack the behavioural features that once defined it
It is likely that in the future, we can add more specific biochemical or molecular parameters to our definition of sleep
Several genes that affect both metabolism and sleep have been identified in mammals
The phylogenetic framework depends on the postulation that sleep states emerged early in animal evolution
Certain functional aspects are probably conserved from an ancestral sleep state
Sleep evolved fundamentally as a metabolic state of the animal and serves functions outside the nervous system
Sleep serves metabolic roles in humans
Some animals sleep more deeply, whereas others extend the duration of their sleep or sleep at inappropriate times
Behavioural response to sleep deprivation provides evidence of sleep’s importance
Sleep during lethargus has been termed developmentally timed sleep
Sleep can be found in an animal that has a mere 302 neurons and that lacks overt circadian behavioural rhythms
Jellyfish show sleep behaviour, demonstrating that neither a brain nor a CNS is required for sleep
Cockroach: when forced to continue locomotion during its resting time, showed an increase in subsequent immobility → suggests homeostatic guarding of the resting behavioural state
Honeybee: reduced arousability was associated with decreased responsiveness of visual interneurons, demonstrating a physiological basis to this behaviour
Fruit flies: prolonged immobility during the night, are less responsive to stimulation during their quiescent period and, following forced movement during the sleep period, display immobility during the early day
Have a clearly defined CNS with a brain with an approximately 24h circadian periodicity
Jellyfish: the frequency of their pulsations is higher during the day than during the night, their responsiveness is reduced at night, and forced movement at night results in reduced activity and reduced responsiveness during the subsequent day
There is more evidence supporting that sleep existed in a common ancestor than that sleep evolved convergently
Evolutionarily distance species share common sleep functions
Claims of a wholly sleepless animal must be viewed with scepticism
REM and NREM sleep occur in mammals and birds and may also be present in reptiles, suggesting that among vertebrates it was present at least as early as when amniotes evolved more than 300 million years ago
Brain neurons are top-down regulators of sleep–wake states in mammals
There is evidence that there is a bottom-up organization
As electrical activity of neurons within local neural groups becomes synchronized with the activity of neurons in other local neural groups, behavioural sleep emerges at the organismal level
The local sleep-like state during wake might interfere with the role of the network in normal wake behaviour and is not as effective as organismal sleep at achieving global sleep functions
When local neural groups eventually become coordinated, the organism shows sleep behaviour
Sleep and wake lie on a continuum
Neither central regulatory neurons nor an intact organism are needed to generate sleep states
The influence of signals from non-neuronal tissues on sleep regulation suggests that the ancestral function of sleep might have resided in non-neuronal cells
Sleep and wake might be controlled by endocrine signals
Non-neuronal cells producing endocrine signals might also be able to influence motor behaviour and reduce sensitivity to external stimuli, resulting in sleep behaviour
There is sleep in animals that lack neurons
If, at its core, sleep were serving a metabolic function, it is not inconceivable that plants, algae and single-cell prokaryotes will also ultimately be considered to sleep
In humans and other terrestrial mammals that are born in an underdeveloped state, sleep need is greatest during development
Increased sleep is apparently coupled to somatic and neural growth and development
Mammals, arthropods and nematodes also all sleep more in the setting of either an infectious or non-infectious illness
Metabolic temporal compartmentalization may serve not only to separate chemically incompatible reactions but also to divert energetic resources used in neural processing to other uses, whether in the brain or in the body
Overall metabolic costs may be reduced by restricting such processes to run at high capacity for a limited portion of the day
The more waste that has been accumulated, the more that can and will be cleared
Emergence of numerous tissue-specific and organism-specific molecular changes during both sleep and wake
Sleep is a time for anabolic activity and the building of macromolecules in all cell types
Within the brain, different regions and cell types show unique patterns of sleep-modulated transcription
Sleep-induced savings in neuronal energy and anabolic metabolism may be conducive to neural plasticity
Sleep emerged early in animal evolution
Metabolic demands are a key driver of sleep regulation
Sleep itself serves primarily metabolic roles
Once the essential function of sleep is understood, it may come to be appreciated in organisms that lack the behavioural features that once defined it
It is likely that in the future, we can add more specific biochemical or molecular parameters to our definition of sleep
Several genes that affect both metabolism and sleep have been identified in mammals
