The researchers examined levels of the neurotransmitter orexin in the cerebrospinal fluid of 60 adults with mild to moderate Alzheimer’s disease over a three-year period.

They found that individuals with elevated levels of orexin, which is vital to sleep and appetite regulation, were more likely to exhibit cognitive decline, including poorer memory and thinking, more severe behavioral and psychiatric symptoms, and higher levels of biological markers associated with neurodegenerative disease and inflammation.

However, that relationship was mitigated by specific brain waves during sleep. Individuals who produced stronger sleep spindles and sleep oscillations—types of brain waves associated with memory support and preservation—during nonrapid eye movement (NREM) sleep had less cognitive decline over time than those with weaker brain waves. This sleep activity appears to provide neural resilience against the negative effects of higher levels of orexin on their cognition and mental health.