Sleeping in cycles within cycles: A nonlinear framework for NREM sleep microstructure

Area of Science:

• Neuroscience
• Sleep Science
• Computational Biology

Background:

• Non-rapid eye movement (NREM) sleep is a dynamic process with fluctuating activity across multiple temporal scales.
• The cyclic alternating pattern (CAP) and infraslow oscillations (ISO) are known markers of NREM sleep instability.

Purpose of the Study:

• To propose a unified framework where CAP and sigma-ISO reflect different aspects of a shared nonlinear regulatory process.
• To reframe CAP as a mechanistic index of NREM sleep regulation.

Main Methods:

• Review and synthesis of existing literature on NREM sleep dynamics, CAP, and ISO.
• Theoretical modeling of the proposed NREM sleep infraslow cycle and its relationship with CAP and ISO.

Main Results:

• CAP is proposed not as a harmonic of sigma-ISO, but as a thresholded output of a broader NREM sleep infraslow cycle.
• CAP-favorable windows occur within the 50-s ISO cycle, potentially explaining the observed 2:1 relationship.
• This coupling is modulated by various factors including ultradian position, autonomic state, and network activity.

Conclusions:

• The NREM sleep infraslow cycle provides a unifying model for understanding NREM sleep instability.
• Phase-resolved multimodal studies are needed to validate this framework.
• Repositioning CAP as a mechanistic index can advance the study of nonlinear NREM sleep regulation.