Sleep is an odd experience – at first glance, it seems nonsensical. Hours of time “wasted” in a vulnerable, supine position where predators can pounce and productivity is at zilch. Of course, over the years, science has revealed the vital health benefits of proper, regular sleep. But as tricky as it may be for some of us to achieve on a daily basis, slumber is a vital process for all animals – or so we thought.

A team from Imperial College London are now challenging that theory in Science Advances, suggesting that for male fruit flies (Drosophila melanogaster), sleep may not be the biological imperative we once thought.

“One of the reasons sleep is considered essential for life is that all animals sleep, so we were surprised to find some flies that needed so little,” said study author Dr Giorgio Gilestro, from Imperial’s Department of Life Sciences, in a statement. “We wanted to find out if this was just a peculiarity for a lucky few flies, or whether it was something any fly could do.”

To keep the flies awake – and this is the controversial bit – the team enclosed them in a glass tube hooked up to an automatic monitoring system that detected when there was no movement for 20 seconds. Should such a lapse of activity happen, the tube rotated and woke up the drowsy fly. 

The scientists found that the sleep-deprived male flies did not die sooner than their well-slept peers. However, for female flies, death occurred 3.5 days earlier. That may not sound significant but it’s about 7 to 9 percent of their average lifespan (40-50 days).

Intriguingly, the females in their day-to-day life actually required less sleep on average than the males, so it’s surprising that they were more sensitive to long-term sleep deprivation, required more time to rebound, and died sooner.

Of course, this study is a controlled experiment in a lab. In the wild, it’s likely that such sleep starvation would have knock-on effects, putting the flies at greater risk of predation. 

"Lack of sleep could make it hard for them to function properly and safely, just as a lack of sleep might cause a sleep-deprived human to crash their car,” added Dr Gilestro.

”It's not that there are no consequences to not sleeping – in fact we will be investigating the effects on mental performance in flies in future experiments – but our study has made us question whether sleep deprivation alone causes death.”

When the flies were allowed to sleep once more, they did sleep for a little longer but returned to baseline one to two days after, not accruing the entirety of the sleep debt they had incurred.

Now let’s return to the controversial part – how the flies were kept awake. Absence of movement is routinely used in science as a proxy for sleep across a wide range of animals. However, a professor who pioneered the study of sleep in flies told The New York Times that she's not convinced the micro-movements the team reported are not part of sleep behavior. It’s possible they could still be getting in micro-naps that are beneficial in some capacity.

In fact, the team themselves concede, “we cannot rule out that, in our sleep deprivation experiments, flies still experience enough sleep to satisfy a hypothetical vital need. That is, prolonged or consolidated sleep is not a vital necessity but micro-intervals of sleep that last only few seconds at the time may be sufficient to satisfy whatever basic biological need that sleep may serve...”

For now, it seems, sleep remains an enigmatic experience.