Newswise — We all know what it feels like when we’ve lost some sleep the night before, whether studying for a big test, binge-watching our favorite TV show, or a late night with friends - it’s tough to concentrate the next day, we feel lethargic, and sometimes our mood is affected too. There’s no doubt that sleep is essential, not just for us humans, but for every animal species. But there are other behaviors that are also essential for an animal’s survival, like eating, drinking, and mating. Being asleep, well, means we can’t do those other behaviors.

“Behind the scenes, the brain is constantly doing a sophisticated cost-benefit analysis of what behaviors to pursue, and which behavior is most beneficial in those circumstances,” says Kyunghee Koh, PhD, associate professor of neuroscience at the Vickie & Jack Farber Institute for Neurosciences and the Synaptic Biology Center at Thomas Jefferson University. “For instance, if an animal is very hungry, it will give up sleep to find food. Similarly, previous work in our lab has shown that a male fruit fly will give up sleep in order to court and mate with a female fruit fly.

In a recent study, Dr. Koh and her lab wanted to know - would food affect how fruit flies make their decision between sleep and courtship? To find out, the researchers deprived some male flies of yeast, an important source of protein. They found that at night, these males chose sleep over sex, unlike their well-nourished counterparts, who preferred to mate. Turns out protein is necessary for any potential offspring to survive. So, these malnourished males chose sleep over courtship.

The researchers then wanted to know – what pathways and circuits in the fly’s brain are involved in this decision-making process. “Unlike humans, who have over 100 billion nerve cells or neurons in our brain, the fly nervous system only has 100,000 neurons,” says Dr. Koh. “It’s still a lot, but it’s much easier to see individual neurons and map connections between them.”

A cluster of neurons called P1 neurons are known to control courtship in male flies. In fact, previous work, including from Dr. Koh’s lab, had shown that turning on P1 neurons not only makes male flies court, but also makes them stay awake, suggesting these neurons may also be involved in regulating the sleep-courtship balance.

To better understand how this worked, the researchers used genetic techniques to trace connections, and found that P1 neurons connect to a pair of neurons that express the chemical dopamine. The researchers then turned on the dopamine neurons and found that the activation suppressed sleep, but only in well-nourished male flies and not protein-deprived males. This suggests that this pathway, in addition to regulating sleep, is integrating cues about the fly’s nutritional status. The effects mimic what they saw earlier – that protein-deprived males did not give up sleep.

“These findings give us a possible pathway to study the complicated analysis going on inside the brain when an animal is choosing a behavior that is optimal for their survival,” says Dr. Koh.

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eLife