Abstract
Sleep is essential in maintaining physiological homeostasis in the brain. While the underlying mechanism is not fully understood, a 'synaptic homeostasis' theory has been proposed that synapses continue to strengthen during awake and undergo downscaling during sleep. This theory predicts that brain excitability increases with sleepiness. Here, we collected transcranial magnetic stimulation measurements in 38 subjects in a 34 hr program and decoded the relationship between cortical excitability and self-report sleepiness using advanced statistical methods. By utilizing a combination of partial least squares regression and mixed-effect models, we identified a robust pattern of excitability changes, which can quantitatively predict the degree of sleepiness. Moreover, we found that synaptic strengthen occurred in both excitatory and inhibitory connections after sleep deprivation. In sum, our study provides supportive evidence for the synaptic homeostasis theory in human sleep and clarifies the process of synaptic strength modulation during sleepiness.
Highlights
During sleep, brains undergo profound neurophysiological changes that restore the decline in cognitive functions associated with sleepiness (Harrison and Horne, 2000; Tononi and Cirelli, 2006)
The present study reported that a combination of four transcranial magnetic stimulation (TMS) parameters could efficiently predict subjective sleepiness
short interval intracortical inhibitions (SICIs) is believed to be mediated by GABAAR, and intracortical facilitations (ICFs) is mediated by glutamatergic transmission, potentially through NMDAR (Van den Bos et al, 2018)
Summary
Brains undergo profound neurophysiological changes that restore the decline in cognitive functions associated with sleepiness (Harrison and Horne, 2000; Tononi and Cirelli, 2006) While this homeostatic process provides an important opportunity in studying the modulation of cognitive states, the key features of neural circuits underlying wakefulness, sleepiness, and sleep remain to be poorly understood. A synaptic homeostasis theory has been proposed to describe the biophysical change of neural circuits during sleep: wakefulness associates with strengthening of the synaptic connection, while sleep initiates synaptic weight downscaling and facilitates homeostasis (Tononi and Cirelli, 2006; Tononi and Cirelli, 2003) This theory reasons that constant learning and memory activities during awake lead to synaptic potentiation, prolonged awake period causes hyperactivity in the neural circuit, enhancing the noise among neural communications and disrupting cognitive functions (Tononi and Cirelli, 2003).
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