Bees learn while they sleep, and that means they might dream

For all our obvious differences, humans and honeybees share some common threads within the fabric of life.

We are both social species. While humans speak and write to communicate, honeybees dance to one another; waggling their bodies for specific durations at angles that indicate where the best pockets of nectar or pollen are to be found outside the hustle and bustle of the nest.

But only forager bees – the eldest of several types of honeybee castes – do this. Just like in human populations, the honeybee colony is divided into different sectors of work. There are cleaners, nurses, security guards, not to mention collection bees whose sole job is to cache nectar in comb.

Different honeybees do different jobs in the colony As they age, honeybees are promoted through a diverse career, from waste disposal to the more familiar forager.

It’s not all work, work, work. Busy bees have to sleep, too

Similar to our circadian rhythm, honeybees sleep between five and eight hours a day. And, in the case of forager bees, this occurs in day-night cycles, with more rest at night when darkness prevents their excursions for pollen and nectar.

But, given that a hive's primary purpose is productivity and yield, why should a large portion of the population seemingly waste up to a third of the day resting? What are the benefits of sleep?

Over the last few years, a handful of scientists have started to uncover why honeybees need to rest; their findings adding to the list of threads that we share.

Ever since Aristotle studied the monarchy of the honeybee colony in the 3rd Century BC, the species Apis mellifera has been studied by generations of dedicated scientists, each able to discover something entirely new.

"The bee's life is like a magic well: the more you draw from it, the more it fills with water," wrote Karl von Frisch, the German Nobel laureate who decoded their waggle dances, in 1950.

It was in 1983 that a researcher called Walter Kaiser made a new discovery: that honeybees slept. As he watched through his observation hive, Kaiser noted how a bee's legs would first start to flex, bringing its head to the floor. Its antennae would stop moving. In some cases, a bee would fall over sideways, as if intoxicated by tiredness. Many bees held each other's legs as they slept.

Kaiser's study was the first record of sleep in an invertebrate. But it was far from the last. The scuttle of cockroaches, the flutter of fruit flies, and the rhythmic undulations of jellyfishes all have temporary periods of quiescence.

"The evidence appears to align with this idea that sleep is shared across all animals," says Barrett Klein, a sleep biologist from the University of Wisconsin Wisconsin–La Crosse. "There's no universally-accepted exception."

Being so prevalent, sleep seems to be a very important part of complex life. To understand why honeybees sleep, a long line of scientists has been keeping forager bees up at night. How do they function sans sleep? Not well, it seems.

Sleepy bees become sloppy

For one, they cannot communicate properly. Instead of performing their waggle dances with incredible accuracy, sleepy bees become sloppy. Their interpretive dances fail to translate the direction of a profitable food source.

And since their nest mates use this information as a guide for their foraging trips, they are likely to be sent slightly astray, wasting time and energy on the wing. The whole colony suffers.

Further, sleep-deprived honeybees find it difficult to return to the hive when visiting fresh flower patches, spending more time reorienting themselves with the sky and surrounding landmarks as their compass. Many even get lost and never return, so their rest becomes much more permanent.

Without a good night's sleep, then, honeybees start to forget the activities that should be second nature to them. And in a study released in 2015, Randolf Menzel and his colleagues from the Free University of Berlin provided a possible explanation as to why this might be.

As is well-documented in humans, deep sleep (known as slow-wave sleep) consolidates memories, transferring them from short-term to long-term memory. Menzel and his team wanted to know whether the same was true for the humble honeybee.

First, they had to teach them something new; only then could they test the quality of their short-term to long-term memory transfer. They chose a tried-and-tested protocol, developed by Menzel himself in 1983.

When feeding, honeybees exhibit a stereotyped behaviour: sticking out their long tubular mouthparts, or proboscis, to slurp up dinner. But, by presenting honeybees with a specific odour and burst of heat as they feed, this proboscis extension response (PER) can be elicited even when there is no food available.

It is the honeybee equivalent of the famous Pavlov's dog response. Rather than a bell, the bees associate the odour-heat combo with food and try to feed.

Only it is much easier to condition bees than dogs. Honeybees are quick learners, associating the odour and heat with food after one to three trials. After that, PER happened without the need for a reward.

Bees are very smart. They are also very sweet to watch while they are learning

"If you work with them, you realise very quickly that they are very smart," says Hanna Zwaka, one of the study's authors. "They are also very sweet to watch while they are learning."

Once conditioned, the bees were allowed a full night's sleep within their own personalised plastic tube. As they slept in solitude, the team exposed some of the honeybees to the conditioned odour-heat combo during different sleep stages, ranging from light sleep to deep sleep, allowing any activity in their brains to be further stimulated.

As a control, a separate group of bees were exposed to a neutral odour – paraffin oil – that would not reactivate any conditioned responses.

When the honeybees woke the next day, the memory tests could begin. Did the bees with the night-time reminders hold on to their conditioned response – sticking out their proboscis – for longer than those without?

When honeybees wake, do they remember?

Yes, but only when the odour and heat were presented in the deep-sleep stage, just like we would expect for a sleep-reinforced memory in humans. Presenting the odour and heat during other, lighter stages of sleep offered no advantage in memory retention.

Learning is reinforced during deep-sleep, just like we would expect in humans.

Although their bodies might be inactive during deep-sleep, honeybee brains do not seem to be. The previous day's activities are reactivated, stabilising fragile memories and converting them into a more permanent form that can be accessed the next day – or perhaps even further in the future.

In sleeping rats, memory consolidation has been shown to work like replaying a tape: any learned responses, such as completing a complex maze, are repeated over and over again in the same sequence that they occurred; right turn by wrong turn, neuron by neuron in the brain.

Menzel and colleagues' study adds some tantalising evidence that the same might be occurring in bees.

"It's a beautifully conducted study with regard to memory," says Klein. But he has some caveats: "Whether or not the results relate to deep sleep is up for discussion." No study has yet clearly demonstrated stages or depth of sleep in insects, he says; only promising hints and suggestions.

Both labs hope to replicate these results with more streamlined, and telling, methods.

With the possibility of memory reactivation in the bees' sleepy heads, Menzel's work begs the question of whether honeybees dream.

In humans, dreams were thought to be a phenomenon of REM sleep, thus limiting the possibility of dreaming to mammals, birds, and (more recently) reptiles; animal groups that exhibit similar eye-fluttering stages of sleep.

But this is not the case. Over recent decades, studies have revealed that dreaming can also occur during slow-wave sleep, the analogue of honeybees' deep-sleep.

When woken from slow-wave sleep, people often recall basic non-narrative dreams such as a house, faces, or a pet. "[Therefore], if bees dream at all, it would be very basic dreaming," says Zwaka. "A special odour, for example. Or a colour of flowers, like yellow or blue."

The magic well of bee biology is still nowhere near empty.

By Alex Riley