A team of neuroscientists at MIT think they know what causes people to zone out when they're sleep deprived. According to a study published this week in the journal eLife, as far as sleep is concerned, the brain is not a binary system. The researchers discovered a brain circuit in the thalamic reticular nucleus (TRN) that triggers small areas of the brain to become less alert or even fall asleep. This neurological phenomenon is known as "local sleep," but it is often referred to as spacing out.

The researchers weakly stimulated the TRN of mice with optogenetics, which MIT News explains is a technique that allows scientists to stimulate or silence neurons with light. The stimulation caused slow waves to appear in a small part of the brain's cortex, and with more stimulation the entire cortex showed slow waves. These slow waves are associated with deep sleep (as well as coma and general anesthesia). After TRN stimulation, the animals start to act like they’re drowsy, lead author of the study, Laura Lewis, said. "They’ll stop moving around, their muscle tone will go down."

The results have given the researchers more knowledge about the TRN's role on the local level as a "gatekeeper for sensory information entering the thalamus," which is important in studies of memory, cognition, and sleep. Understanding the brain's arousal mechanisms could be instrumental to developing sleep aids and anesthetics that better mimic natural sleep.