In the study, participants were asleep when they wore the device: a head cap embedded with electrodes that measures shifts in fluid within brain tissue, the neural activity from sleep to wakefulness and changes in the brain’s blood vessels.

By measuring these three features, the researchers found they could monitor the brain’s glymphatic system, which acts as a waste-removal and nutrient-delivery system.

This is the first time that researchers have been able to track the flow of glymphatic fluid in individuals at different levels of sleep through a single night. Previously these processes could only be monitored at university research centers by using MRI, an approach that is too slow to track minute changes in individuals’ sleep stages.