by Jamilla Akhund-Zade
figures by Rebecca Senft

Anyone who has had the misfortune of missing a night of sleep would not need convincing that sleep is important for well-being. Decades of studies concur with what is already commonly known – sleep is vital for the health of the body and the brain, and lack of sleep can be deadly. Yet contrary to our current understanding, a recent study of the humble fruit fly, Drosophila melanogaster, shows that sleepless flies continue to lead normal lives.

The importance of sleep

The average person spends approximately one-third of their time sleeping and, fortunately, that time is not wasted. Sleep has direct benefits for memory and learning,  metabolism and hormone balance, waste product removal from the brain, and emotional health. In order to study whether sleep is absolutely vital to survival, researchers have used model organisms to track the effects of sleep deprivation on health and survival (Figure 1). In rats, for example, sleep deprivation over the course of two to three weeks is lethal. In humans, those affected by Fatal Familial Insomnia, a rare genetic disorder caused by misfolded proteins in the brain, suffer from an inability to sleep and die within months of showing symptoms.

While these sleep deprivation experiments seem to point to the fact that going without sleep is deadly, there still exists room for speculation about the necessity of sleep. In pigeons, chronic sleep deprivation did not cause the same lethal effects as in rats.

In the case of fatal familial insomnia, it is difficult to say whether death occurs due to lack of sleep or due to the destruction of neurons caused by the disease – infecting mice with the same misfolded protein that is responsible for Fatal Familial Insomnia causes premature death without interrupting sleep.

Sleepless flies survive

Given the potential that sleep deprivation may not be deadly, scientists from the Imperial College London decided to rigorously investigate the sleep habits of the fruit fly in order to see if there is a link between sleep deprivation and survival. Fruit flies are a common organism used in laboratory studies – they are easy to maintain, and many tools have been developed over the years to study their genetics, neurobiology, and behavior. The researchers housed flies in individual vials with food and recorded the flies’ behavior using computer-assisted tracking and machine learning. These two approaches allowed them to record every single position of the fly in the vial, but also to differentiate between sleep (immobility lasting longer than five minutes) and what they term ‘micro-movements’ or movements that happen while the fly is standing in one place: grooming, egg-laying, and feeding.

They found that when looking at normal healthy flies of the same genetic strain, sleep amounts were very different between individuals – some flies were only reported to sleep a few minutes a day while others slept for 10 hours. Their surprising finding showed that some flies are naturally sleepless, since their sleep habits remained the same when tested again in a new tube.

Having seen that there are naturally occurring sleepless flies, the authors of this study investigated whether depriving flies of sleep would decrease their survival. They monitored the flies using their computer-assisted tracking and as soon as a fly was still for 20 seconds, the tube would spin to interrupt their sleep. This persisted for the entire lifespan of the fly, which can be almost two months in a laboratory environment. Even though the sleep-deprived flies showed a 95% reduction in the amount of sleep they got, there was only a slight decrease of a couple of days in the average lifespan of the male and female flies (Figure 2). Even with a severe reduction in sleep – one that proves deadly to rats and humans – flies do not seem to be perturbed.

 

Sleepless flies and us

The authors’ conclusions seem to go against the current thinking about the importance of sleep – how is it that flies can survive on just a few minutes of sleep a day? The authors propose a theory that the amount of sleep can be divided into three components – a vital one, a useful one, and an accessory one (Figure 3). They draw a parallel with another important behavior for survival – eating. When you eat, there is a certain number of calories you must intake to survive (vital), there is an amount you intake for better function (useful), and there is an amount you intake just because the food is tasty (accessory). They argue that their sleep deprivation experiments removed the accessory and useful sleep, but flies were able to fulfill their vital sleep needs just by sleeping in tiny bouts of a few seconds.

While such short sleep times sound shocking (or perhaps alluring) to us, there is a wide variation in sleep amount across different species. Brown bats sleep 20 hours a day, while large migratory herbivores such as elephants, giraffes, or horses sleep less than 3 hours a day. Migratory birds barely sleep on their long journeys. It is possible that this variation in sleep amount reflects an adaptation to the surroundings or lifestyle of the animal that optimizes their energy usage (looking for food vs. sleeping) and reduces risk of death by accident or predator. Therefore, even for animals that sleep for long periods, the majority of that time may be attributed to the useful or accessory components of sleep rather than the vital. Humans also have natural variation in sleep amount, with some short sleepers requiring only six hours or less of sleep a night to function well, though for the majority getting less than eight hours of sleep a night detrimentally affects mental performance.

Evidence from fruit flies may be encouraging to those of us that would like to stay up into the wee hours of the morning, but sadly, the risks of not sleeping for humans still greatly outweigh any potential benefits. For now, it is advisable to get some shut-eye and leave the all-nighters to the flies.

Jamilla Akhund-Zade is a fifth-year Ph.D. candidate at Harvard University who studies thermal preference in fruit flies.

Rebecca Senft is a fourth-year Program in Neuroscience PhD student at Harvard University who studies the circuitry and function of serotonin neurons in the mouse.

For more information:

• For an insightful look into the nature of sleep and sleep disorders, check out this National Geographic article by Maggie Steber.
• To read the original research on sleepless fruit flies by Geissmann, Beckwith, and Gilestro discussed in this article, look at their open-access pre-print on bioRxiv.
• For recent research on how fruit flies are used to understand the mechanisms of sleep, check out this work from Johns Hopkins.