* Scientific study references that back up my points can be clicked on in the text to open the original reports in a separate tab. They can also be found at the bottom of the article.Many of us have at some point heard the story of dolphins sleeping with one brain hemisphere at a time. This partial sleep phenomenon, whereby some regions of the brain appear to exhibit properties of wakefulness while others submerge themselves into deeper sleep (referred to as slow-wave sleep), is actually quite common across the animal kingdom. Many aquatic mammals, birds, and possibly reptiles can sleep with one eye open, while the hemisphere connected to that eye remains in a wake-like state, as if on a lookout for threat.
Indeed, the consensus amongst biologists appears to be that partial brain sleep has an essential safeguarding purpose, allowing animals to remain vigilant of their surroundings and capable of reacting to potential threats at short notice. This is supported by observations of groups of wild ducks, where individuals lying on the edges of a sleeping flock have a habit of keeping an eye open away from the group centre – where predators are most likely to emerge. Researchers have found that this tendency declines significantly towards the centre of the flock, where roughly 88% of ducks indulge in sleeping with both eyes shut, as opposed to the relatively fewer 69% of individuals on the edge of the group.
Furthermore, recording these ducks’ electrical brain activity revealed that, while the hemisphere connected to the closed eye exhibits classic slow-wave activity (a hallmark of deep sleep), this phenomenon is significantly weaker in the hemisphere that receives inputs from the open eye. This indicates that, in one-eye-open sleepers, one hemisphere remains in a ‘quiet waking state’. This, it turns out, enables surprisingly quick escape reflexes! When ducks sleeping with one eye open were shown videos of expanding images to mimic the approach of a predator, it took them on average only 165 milliseconds to make a sudden dash for safety.
It’s clear that preventing the entire brain from becoming consumed by deep sleep can pay off when animals live in risky environments. Recently, the journal Current Biology published some evidence that a similar phenomenon might actually be happening in our own brains the first time that we sleep in a new and unfamiliar place. Under these circumstances, humans have a well-documented tendency to take a little longer to doze off and have more fragmented sleep than normal. The new findings indicate that this first-night effect might be underpinned by the fact that parts of our left hemisphere remain on night watch when we fall asleep some place new. Ultimately, degraded sleep quality is likely a small price to pay for having a brain that temporarily remains more alert just when it counts the most – that is, when we shut our eyes and minds on new and uncertain places, full of potential threats.
In this experiment, researchers examined the brain activity and eye movement patterns of their participants during the first and second nights of sleeping in an unfamiliar room, using recording electrodes placed at various sites on the scalp and around the eyes.
Their analyses revealed that the typical sleep disturbances of the first night were associated with some peculiar effects in the so-called default mode network of the brain. This term has been given to a collection of brain regions that are consistently found to be active whenever people lying in a brain scanner aren’t asked to do anything in particular and are left to their own devices. Researchers don’t pretend to know what exactly human minds get up to when there is nothing to do – they could be contemplating life, thinking about what’s for dinner, or mentally replaying an awkward conversation from earlier. But whatever the mental state is, the fact that it appears to be associated with activity in the same set of brain regions across many brain scanning experiments has given rise to the idea that these regions represent a fundamental network that acts as some sort of scaffold for our thought processes. So what happened to the default mode network when participants entered a state of deep sleep for the first time in a new place?
It turned out that this network produced substantially weaker slow-wave activity in the left hemisphere compared to the right. This asymmetry indicated that parts of the left hemisphere remained in a much lighter sleep state than the rest of the brain, and this was not without its consequences. The experimenters found that the less slow-wave activity an individual’s left hemisphere produced compared to the right, the longer they took to fall asleep. This indicates that our tendency to have more fragmented sleep in new places might have something to do with the fact that parts of the left hemisphere maintain a certain level of vigilance even as the rest of the brain plunges into deep sleep.
This phenomenon likely served a critical threat-monitoring purpose at times when humans lived in far less sheltered environments. Consistent with this possibility, the study found that individuals’ left hemispheres (and the individuals themselves) were surprisingly reactive to unexpected sounds that occurred when they first entered deep sleep in the new place.
To show this, the researchers used a so-called oddball paradigm, which involves playing a sequence of identical beeps roughly one second apart, peppered with the occasional ‘oddball’ beep that stands out from the rest in terms of pitch.
Such rare and unexpected inputs normally produce an electrical brain response whose strength depends on the alertness of the listener. Now, we know that information from one ear is primarily received by the hemisphere on the opposite side of the brain. This allows us to reason that playing such a sequence of beeps separately into the right and left ears of sleeping individuals could help us test just how alert the left hemisphere is compared to the right, which shows greater levels of slow wave activity – a hallmark of deep sleep. As you might have already expected, the experimenters found that the strength of the brain response to oddball beeps was substantially boosted in the ‘light sleeper’ left hemisphere compared to the right. This effect was seen exclusively on the first night that individuals slept in the new place – by the second night, any difference in sound reactivity between the left and right hemispheres was eradicated.
It turned out that the left hemisphere’s heightened sensitivity to unexpected sounds on the first night had some important consequences for behaviour. Before going to bed, some participants were asked to lightly tap their fingers whenever they heard a sound in their sleep. The researchers found that these individuals were significantly more likely and quicker to wake up upon hearing an oddball beep on their first night in the new place compared to the subsequent night – an effect that was even stronger for beeps that were played to the right ear, which sends signals mainly to the ‘light sleeping’ left hemisphere. The fastest awakenings were observed in individuals whose left hemispheres produced the weakest slow wave activity compared to the right.
When we look at this evidence, it appears likely that our tendency to struggle with dozing off and to experience shallow sleep when we first settle in new places might be a by-product of the fact that parts of our left hemisphere don’t enter a state of deep sleep even when the rest of our brain does. This allows some brain regions to remain more responsive to our surroundings, poised to trigger reactions at a moment’s notice when we make ourselves most vulnerable by shutting off our consciousness in unfamiliar and possibly dangerous environments. After all, until very recently humans didn’t have beds to sleep in. In this respect, maybe we’re not so different from ducks. References
- Tamaki, M. et al. (2016). Night watch in one brain hemisphere during sleep associated with the first-night effect in humans. Current Biology.
- Buckner, R. L. et al. (2008). The brain’s default network. Anatomy, function and relevance to disease. New York Academy of Sciences.
- Manoach, D. S. and Stickgold, R. (2016). Sleep: keeping one eye open. Current Biology Dispatches.
- Rattenborg, N. et al. (1999). Half-awake to the risk of predation. Nature.
- Rattenborg, N. et al. (2000). Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep. Neuroscience & Biobehavioral Reviews.
- Siegel, J. M. (2005). Clues to the functions of mammalian sleep. Nature.
- Tamaki, M. et al. (2005). Examination of the first-night effect during the sleep-onset period. Sleep.