Homeostatic, circadian and ultradian mechanisms play crucial roles in the regulation of sleep. Evidence suggests that ratios of low-to-high frequency power in the electroencephalogram (EEG) spectrum indicate the instantaneous level of sleep pressure, influenced by factors such as individual sleep-wake history, current sleep stage, age-related differences and brain topography characteristics. These effects are well captured and reflected in the spectral exponent, a composite measure of the constant low-to-high frequency ratio in the periodogram, which is scale-free and exhibits lower interindividual variability compared to slow wave activity, potentially serving as a suitable standardization and reference measure. Here we propose an index of sleep homeostasis based on the spectral exponent, reflecting the level of membrane hyperpolarization and/or network bistability in the central nervous system in humans. In addition, we advance the idea that the U-shaped overnight deceleration of oscillatory slow and fast sleep spindle frequencies marks the biological night, providing somnologists with an EEG-index of circadian sleep regulation. Evidence supporting this assertion comes from studies based on sleep replacement, forced desynchrony protocols and high-resolution analyses of sleep spindles. Finally, ultradian sleep regulatory mechanisms are indicated by the recurrent, abrupt shifts in dominant oscillatory frequencies, with spindle ranges signifying non-rapid eye movement and non-spindle oscillations – rapid eye movement phases of the sleep cycles. Reconsidering the indicators of fundamental sleep regulatory processes in the framework of the new Fractal and Oscillatory Adjustment Model (FOAM) offers an appealing opportunity to bridge the gap between the two-process model of sleep regulation and clinical somnology.

中文翻译：

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睡眠调节的基础知识：分形和振荡神经动力学的模型和基准值

稳态、昼夜节律和超昼夜机制在睡眠调节中发挥着至关重要的作用。有证据表明，脑电图 (EEG) 频谱中低频与高频功率的比率表明睡眠压力的瞬时水平，该比率受到个人睡眠-觉醒历史、当前睡眠阶段、年龄相关差异和大脑地形特征等因素的影响。这些效应在频谱指数中得到了很好的捕捉和反映，频谱指数是周期图中恒定的低高频比的综合测量，它是无标度的，并且与慢波活动相比表现出较低的个体间变异性，有可能作为合适的标准化和参考措施。在这里，我们提出了一个基于光谱指数的睡眠稳态指数，反映人类中枢神经系统的膜超极化和/或网络双稳态水平。此外，我们提出了这样的观点，即振荡的慢速和快速睡眠纺锤波频率的 U 形夜间减速标志着生物夜晚，为睡眠学家提供了昼夜节律睡眠调节的脑电图指数。支持这一论断的证据来自基于睡眠替代、强制去同步协议和睡眠纺锤波高分辨率分析的研究。最后，超电睡眠调节机制是通过主振荡频率的反复、突然变化来表明的，纺锤波范围表示非快速眼动和非纺锤波振荡——睡眠周期的快速眼动阶段。在新的分形和振荡调整模型（FOAM）框架下重新考虑基本睡眠调节过程的指标，为弥合睡眠调节双过程模型和临床睡眠学之间的差距提供了一个有吸引力的机会。