Wide-spread brain activation and reduced CSF flow during avian REM sleep

Abstract Mammalian sleep has been implicated in maintaining a healthy extracellular environment in the brain. During wakefulness, neuronal activity leads to the accumulation of toxic proteins implicated in Alzheimer’s disease. The glymphatic system is thought to clear these proteins by flushing cerebral spinal fluid (CSF) through the brain. In mice, this process occurs during non-rapid eye movement (NREM) sleep. And, in humans, increased ventricular CSF flow during NREM sleep, visualized using functional magnetic resonance imaging (fMRI), is also thought to be coupled to flow through the glymphatic system. But does waste clearance occur throughout sleep or is it specific to NREM sleep? Using fMRI of naturally sleeping pigeons, we show that REM sleep, a paradoxical state with wake-like brain activity, during which we experience our most vivid dreams, is accompanied in birds with the activation of brain regions involved in processing visual information, including optic flow during flight. We further demonstrate that, compared to NREM sleep, REM sleep is associated with a sharp drop in ventricular CSF flow. Consequently, functions linked to brain activation during REM sleep might come at the expense of waste clearance during NREM sleep.