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Sleep and sex are, for the most part, mutually exclusive activities.* If someone is simultaneously tired and horny, they have to choose between satisfying two competing drives. A new, surprisingly interesting study from Thomas Jefferson University sheds light on the sex-vs.-sleep battle in fruit flies. It turns out that male flies pull all-nighters in order to court eligible lady flies, much like nerdy protagonists in a '90s teen sex comedy. The study also unearths the biological basis of this behavior — researchers discovered a bundle of neurons, called MS1 neurons, that give male flies the jolt they need to stay up and flirt the night away.

Intuitively, it makes sense that there's a trade-off between sleep and sex — as well as between sleep and other waking activities. But the biology underlying these sorts of choices isn't well-understood. "To my knowledge," lead study author Kyunghee Koh said via email, "this is the first experimental study of the competition between sleep and sex drives. Our work provides a valuable entry point for research on competing drives by identifying specific neurons involved in balancing two essential drives."

Fruit flies (technically called drosophila) are often used as a model for human sleep because they appear to share certain features of mammalian sleep, such as circadian patterns. And the winged study subjects were particularly useful in the current study because the key brain chemical underlying their sex-and-sleep behavior, called octopamine, is analogous to norepinephrine, a wakefulness-promoting neurotransmitter in humans.

"Fly sleep is a lot like human sleep," said Koh. "It is already known that norepinephrine (which is similar to octopamine) promotes wakefulness in humans. Our work suggests that norepinephrine may be involved in balancing sex and sleep drives in humans." 

Octopamine, researchers found, is released by the aforementioned MS1 neurons, which basically function like Redbull-Vodka for flies. But MS1 don't work alone. They team up with a group of neurons, called FRU neurons, which regulate sexual behavior in flies. It's a joint effort to keep male flies awake (thanks, MS1) so they can spit mad game (thanks, FRU). 

"There is extensive mutual excitation between MS1 and FRU neurons," said Koh. "While FRU neurons are the key regulators of male sexual behavior, flies have to be awake for FRU neurons to do their job. MS1 neurons keep male flies awake under the right circumstances."

And what are those circumstances — after three glasses of wine? on the third date? Not quite, but male flies don't forego sleep for sex all the time. Researchers ran a few different experiments to gather intel on the late-night behavior of flies, as well as to elucidate the MS1-FRU mechanism.

First they monitored sleeping, waking and courtship activity in pairs of same-sex and opposite-sex flies. In general, they found, males suppressed sleep when two conditions were satisfied:

a) Males were hanging out with females. Male flies got far less nighttime sleep when they were in the presence of female flies than when they were with other males. And the males devoted most of their up-at-night time to courting females.

and

b) Males had a high sex drive. When male flies had low sex drives, they were able to sleep in the presence of female flies. Two things lowered their sex drives: being sexually satisfied and being severely sleep-deprived. This is similar to how humans work — when someone has just had sex or has been awake for 36 hours, they're probably going to choose hitting the sack over getting frisky. 

Then researchers looked at the brain activity underlying flies' sleep-vs.-sex decisions. They did this by both measuring natural MS1 activity during sleep and courtship and artificially manipulating MS1 neurons to see if and how flies' behavior changed in response. 

MS1 neuronal activity was always high, researchers found, when males suppressed sleep to court females. Basically, they needed active MS1 to go on the prowl at night. And, when researchers artificially activated MS1 neurons in males, they slept 75 percent less than male flies in the control group. By contrast, when they silenced MS1 neurons in males, they slept more and courted less than the control males did. 

Neither activation nor silencing of MS1 neurons, however, had any effect on sleep in females. It's not that MS1 neurons themselves are different in males and females, researchers explained in the study, but rather that MS1 and FRU neurons are connected differently. And this is where Koh and colleagues plan to pick up their work. "The next steps," Koh said, "will be to investigate how females balance sleep drive and their reproductive needs."

Additionally, Koh and colleagues are interested in pinpointing the specific FRU neurons that communicate directly with the MS1 neurons. Stay tuned for updates on the sleep-less sex lives of drosophila. 

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*If you're a parasomniac who walks, talks or has sex in your sleep, then you're arguably an exception to this rule.