An intriguing new study, from a team of Swiss researchers, has revealed neural activity during REM sleep in a particular region of the brain known to affect appetite and feeding behaviors significantly influences waking eating patterns. Despite a hefty volume of robust study, REM sleep is still a mysterious and unique sleep phase. Named after the rapid eye movements that occur in all mammals during this sleep phase, it has also been referred to as paradoxical sleep, due to the strange similarity in brain activity between waking states and REM sleep. The new research homed in on a brain region called the lateral hypothalamus. This tiny brain region, found in all mammals, is known to play a fundamental role in food intake, compulsive behavior, and a number of other physiological processes. Previous studies have detected high neuron activity in the lateral hypothalamus during REM sleep, but exactly why this would be happening has been unclear. So to better understand exactly what function this REM-induced lateral hypothalamus activity is serving, a new study used a rodent model to explore what happens when this neural activity is blocked. Using optogenetics, a technique where light pulses can either activate or inhibit the activity of specific neurons in animals, the researchers were able to entirely block lateral hypothalamus activity during REM sleep, and only during REM sleep. The results surprised the researchers. Not only did this intervention lead to the animals consuming less food in their waking hours, but a single night’s intervention had lasting effects on eating behaviors for several days. “We were surprised how strongly and persistently our intervention affected the neural activity in the lateral hypothalamus and the behavior of the mice” says first author on the new study, Lukas Oesch. “The modification in the activity patterns was still measurable after four days of regular sleep.” It’s unclear what role this newly discovered mechanism actually plays in regulating an organism’s day-to-day feeding patterns, but the researchers hypothesize some kind of “sleep reset” mechanism may be taking place, whereby day-to-day fluctuations in lateral hypothalamus activity is recalibrated during REM sleep. Further research will have to better elucidate how REM sleep disruptions influence eating patterns in human subjects, and whether there are ways to modulate dysfunctional eating patterns by tinkering with this REM sleep mechanism.