Even in deep sleep, the brain remains alert.

Even while asleep, fruit flies aren't necessarily easy prey: Even while slumbering, the animals can perceive and react to strong stimuli, as researchers from the Charité report in the journal "Nature." Observations of fruit flies (Drosophila)—better known as fruit flies—may also provide clues to how we humans react to strong stimuli while sleeping.

For many living beings – including humans – this is a fragile balance: On the one hand, we need sleep for regeneration and memory formation, for example; on the other hand, we must be able to perceive emerging dangers even while asleep in order to respond. Exactly how the organism balances this remains unclear, the researchers write.

The team led by neurophysiologist David Owald examined what exactly happens in the brain during these processes in fruit flies. Central to this are two brain networks that process visual stimuli – one activates the response to stimuli, the other inhibits them. "When both networks are active simultaneously, the inhibitory network wins and the processing of the stimuli is blocked," explains co-author Davide Raccuglia. "The fly thus gently blocks out its surroundings and can fall asleep."

However, the activating network is not completely suppressed: "We discovered that the brain of flies finely tunes activating and inhibitory networks during sleep," emphasizes Owald. Particularly strong stimuli are allowed through. "The condition is comparable to a window left ajar: The airflow, i.e., the transmission of stimuli, is interrupted, but a strong gust of wind can push the window open, and a strong stimulus can thus wake the animal."

Specifically, according to the study, this probably occurs in windows between slow, synchronous electrical waves – so-called slow waves – that arise in brain networks when falling asleep. The waves arise because the electrical voltage of the nerve cells oscillates up and down once per second. "It's possible that when the voltage is high, a brief period occurs during which information can pass through the sleep filter," explains co-author Raquel Suaréz-Grimalt.

And how does it work for us? "In humans, we know of a structure in the brain that filters stimulus information and acts as a rhythmic pacemaker—that's the thalamus," says Owald. "There could be parallels here to the processes in the fly brain; perhaps these actually reflect a universal principle of sleep." However, this needs to be researched.

The Charité team is using the fruit fly to research fundamental decisions, such as eating or sleeping. With approximately 200,000 neurons, flies have a relatively compact brain. Some behaviors and brain processes observed in them can be transferred to humans in a slightly modified form.