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  • Writer's pictureDave Black

Sleeping in the Job

Do honey bees sleep? Almost certainly so, explains science writer Dave Black, but what do they achieve in sleep and how does that compare with other animals and humans? Here’s what we know, and how you might be able to spot a slumbering bee.

By Dave Black

All animals sleep, but there the similarity ends. Or least all the multi-cellular animals that have been studied do, including sponges and jellyfish, worms, molluscs, insects, reptiles and amphibians, and birds and mammals. Sleep is not conditional on a brain. Now that could be because we haven’t defined what it means to ‘sleep’ very well, (how does sleep differ from rest, or being comatose?) or possibly that sleep just happens, another name for when nothing happens and there is nothing to do, the clock is just running.

Worker honey bees displaying typical sleep postures while relatively immobile, with limbs and body drooping in the direction of gravity. Bees exhibit a sleep state while (A) in groups, (B) isolated, (C) dangling motionless from tarsal claws, or (D) leaning against the observation hive wall or floor.

Sleep is different from rest, and not the same as a coma either. Our own experience knows it to be different, and when we look at other animals we recognise the immobility, reduced responsiveness, and reversibility that characterise sleep. We see dogs and children sleeping but clearly alive, and science can monitor electrical signals and body temperature or chemistry that mark ‘sleep’ and not ‘awake’, so definition isn’t the problem.

If sleep wasn’t important we would expect to find animals that don’t sleep (we haven’t, although there have been some marginal cases), and that not sleeping would not have any harmful consequences (it does, even for bees). The genes at work are found in lots of other animals too. Most scientists, then, agree that that for all these diverse animals there must be some fundamental function or functions that sleep provides, they just don’t agree what that is. Yet.

Somnolent bees

Honey bees sleep, although that’s been contentious for the last hundred years. Observations in glass hives see bees, at least, resting. They look mostly immobile, but movement in their abdomen shows they are breathing. They have a characteristic posture, ‘slumping’ a bit down onto the surface of the comb. Their antennae retract a little and droop down. Puffs of air on the eyes will ‘wake’ them briefly.

In 1983 Walter and Jana Kaiser measured electrical signals from optical nerves in forager bees and found a circadian response rhythm and reduced sensitivity (an increased ‘response threshold’) to moving patterns near their eyes when they slept. If they are prevented from sleeping (just keep shaking them every so often) they exhibit ‘sleep-deprivation’ by subsequently imprecise waggle-dancing, a greater tendency to get lost on the way home, forgetting tasks they were trained to do, and sleeping for longer when given the chance.

One insect studied in much more detail than honey bees have been is the fruit-fly Drosophila. The reason it is so closely studied is that it is possible to compare what is known about its biology and genetics to other animals, including other insects, and even people. In the last twenty years experiments with Drosophila have demonstrated pretty clearly the neuronal and genetic ‘mechanics’ for sleep and there is enough evidence to believe the same mechanisms are broadly applicable to honey bees too. So, honey bees do sleep.

Our picture is a little more complicated because of the honey bee ‘caste’ organisation, so not every individual bee behaves in quite the same way.  Broadly, the bees that have not been outside sleep differently to the ones that do go outside; no surprises there. Younger bees generally slept inside vacant cells, not outside them, and closer to the centre of the nest. As they age and change tasks they gradually spend less and less time sleeping inside cells. Older worker bees generally sleep in cooler locations nearer the edge of the nest and away from uncapped brood. The average surface temperature of sleeping foragers themselves was lower than the surface temperature of their surroundings, another indicator of sleep.

Drosophila, a much-studied fruit fly from which observations about other animals – including honey bees – can be made.

Special, but not unusual

 One other thing makes honey bees quite different from Drosophila. They have a ‘home’, a nest that is always in the same place filled with combs and food but which is not uniform or unchanging. Interestingly with Drosophila, environmental and social enrichment modify sleep patterns, and so does the size of the group. Group size is probably just a form of enrichment; group members inevitably interact and the added complexity of a larger group is itself ‘enriching’). I’m saying ‘interesting’ because honey bees are large groups in rich environments, but also because it could be a clue to what the purpose of sleeping might be, and because the same thing happens to me! If sleep achieved one biological function (like recuperation) you would think to an observer the behaviour would appear quite uniform. Instead, particularly for animals in complex, rich environments (me at a party!) subsequent sleep is more lengthy, and marked by several periods of different behaviours.

Do Honey bees dream?

What purpose, could sleep serve? It’s been thought for a long time that sleep is a form of rest; at a physical and neurological level a time to repair, re-fuel, and reset essential cellular processes, and a chance to sort, rank, and file experience. It’s not clear that sleep is essential for these acts, or whether they merely make use of an opportune moment. It does seem plausible that different things happen in the different phases of sleep animals experience. For example, a deep or ‘quiet’ sleep may have an effect at a cellular level, allowing the elimination of waste materials, or ‘topping-up’ energy stores. An ‘active’ sleep phase might have more to do with processing the day’s encounters. Honey bees have been shown to demonstrate different behaviours corresponding to different sleep phases, including an ‘active’ or ‘wakeful’ stage, and given what we know from Drosophila, it’s apparent small insect brains are at least as complex as higher animals like us when it comes to sleeping.

A fascinating article from 2021 (classified as ‘Hypothesis and Theory’)[i] proposes that the job of an animal’s brain is to be a ‘prediction machine’. A brain in active sleep is building and rebuilding models about its world to optimise an ability to predict the next event. Sleep periodically detaches a brain from the real world, allowing it to ‘freewheel’ through a greater range of predictions and outcomes than it would normally encounter (we might call it ‘imagine’). An incorrect prediction (like a ‘surprise’) produces an ‘emotional’ response, an error correction mechanism that refines predictions by carrying value - a ‘good’ error, or a ‘bad’ error, or a ‘meh!’. In an environment where change is inevitable an on-going tension between successfully predicting what happens next, but preserving surprise at novelty, is an essential adaption that keeps organisms aware, attentive, and learning. A honey bee’s dreams may be doing exactly that, updating their model of life.

References

[i] Van De Poll Matthew N., van Swinderen Bruno, (2021) Balancing Prediction and Surprise: A Role for Active Sleep at the Dawn of Consciousness? Frontiers in Systems Neuroscience. https://www.frontiersin.org/articles/10.3389/fnsys.2021.768762


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