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Sleep is a variable state depending on species, developmental stage, available physiological markers:
· Kimberly Whitehead talk: In preterm infants, the active sleep stage fulfills a development function that is distinct from the proposed functions of sleep in full-term infants and adults. Namely to develop the somatosensory maps in the brain for processing external inputs like the barrel formations in whisker barrels.
· Jerry Siegel talk: Sleep in different species doesn’t show same physiological markers as in usual lab animals and humans. Dolphin and killer whale unihemispheric slow-wave sleep – is it the same as human sleep? Brown Bat sleeps ~20h per day – is that the same state as other animals? Can we relate all animal “sleep” states to human sleep states?
· Summary: the question of what is sleep is not fully answered. Sleep is different in different life stages and different species
What are key physiological processes that contribute to sleep timing, duration?
· Jerry Siegel talk: Body temperature has large effect on sleep durations, especially REM sleep occurrence. Should be considered in models
· Beth Klerman talk: Circadian rhythm acts to consolidate sleep and wake episodes. At end of the wake episode, circadian rhythm generates a wake-promoting signal, the no-sleep zone. This acts to keep us awake longer so that when sleep occurs it lasts longer and is more consolidated. Similarly, the circadian sleep-promoting effects are strongest at the end of the sleep period, so that we stay asleep longer and the subsequent wake episode will be more consolidated. Also has new paper coming out in J Biol Rhythms on the phase shifting effects on circadian rhythm of very short light exposure in the dark period
· Cecilia Diniz Behn talk: Signalling of SCN to sleep-wake centers may not be 1-dimensional. In rat, requires both sleep-promoting and wake-promoting effects to account for SCN lesion data. In squirrel monkey, can be 1-dimensional effect perhaps because homeostatic time constants are longer relative to circadian rhythm than in rat.
Role of EEG signatures of sleep, such as spindles. Ripples, slow oscillations, theta range oscillations, depends on developmental stage and type of EEG event
· Kimberly Whitehead talk: In neonates, twitches are initiated in periphery and cause a spindle-burst that propagates throughout the somatosensory cortices. This is a signature of propagating activity throughout brain regions and contributes to constructing the neural map of the sensory system. In neonates, sleep oscillations subserve sensory cortical organization
· Sara Aton talk: Different oscillations occur in cortex and hippocampus in different sleep states. In NREM, have slow oscillations (different than slow waves) in cortex and have sharp waves and ripples in the hippocampus. In REM, cortex shows asynchronous activity and hippocampus shows theta oscillations. Oscillations are the substrate for temporal coding of information. The relative phase of cell firing to the oscillation peak sets the temporal frame for temporal coding. A rate code during wake and shift to a temporal code during sleep oscillation with most excitable cells firing first during the oscillation. The temporal code during sleep oscillation also support appropriate patterns of synaptic plasticity for memory consolidation.