🐬 How some animals can sleep half a brain at a time

When some aquatic mammals like dolphins and whales fall asleep, something unusual happens… As they continue to drift through the water, one half of their brain remains fully awake and active while the other side falls into deep, restful sleep.

When fully awake both halves of the dolphin brain work together, but during sleep they uncouple and operate independently. And when one side of the brain has fully rested up, they switch, allowing the other side to recover.

“Uni-hemispheric Slow Wave Sleep” (USWS), a.k.a split-brain sleep, isn’t just restricted to dolphins and whales though - birds take it to another level.

When some species of birds sleep as a flock, something really interesting happens. When they line up in a row, most can sleep with both brain halves. But the birds at either end of the line have the responsibility of standing guard. They will fall asleep with just one eye (and one half of the brain) staying awake. This means both the left and right side of the flock are protected. During the night, the end-guards will turn around so their other eye (and other brain hemisphere) can get some sleep too. This synchronised rotation maximises rest while maintaining group safety.

🌓 The science behind why you often sleep worse in hotels

Like most mammals, it has long been thought that humans sleep completely bi-hemispherically (with both sides of the brain asleep together). However, recent studies increasingly suggest that this might not entirely be the case.

This 2016 study published in Current Biology found that when we try to sleep in unfamiliar circumstances, both sides of the brain don't fall into sleep as they do when you sleep at home.

In their experiments, they found that the left hemisphere stays more alert and participants increasingly struggled to get deep, restorative sleep. The researchers explained this asymmetrical disruption as being linked to the so-called the "first-night effect".

This is the reason that sleep researchers often have to include an “adaptation night” in their studies, when they place participants in a sleep laboratory. One half of the brain sleeps lighter than the other, almost as if it is standing guard. We are never more at risk than when asleep and some scientists have speculated that this is the brain’s way of registering a risk and keeping vigilant to deal with threats at short notice.

🏨  Tips to beat the “first-night effect” when sleeping away from home

We’ve known for a while that brainwave patterns during sleep can be asymmetrical. There is limited research on the matter, but we’re now beginning to see that these asymmetries can also be affected by external stimuli.

In one instance of researchers trying to investigate the cause of the first night effect, they used several methods to precisely measure brain activity over two nights of sleep, a week apart, among a total of 35 volunteers. They consistently found that on the first night, a particular network in the left hemisphere remained more active than in the right hemisphere, specifically during the deepest sleep phase. A factor that made this worse? Sound.

Additionally, another study found that stimulating one eye of a subject with bright light (1,500 lux) before bed would suppress certain types of brainwave activity in the linked brain hemisphere. (For example stimulating the right eye would disrupt brainwave activity in the left hemisphere of the brain.)

While we’re still not entirely sure how it all works, it’s thought that modalities such as light and sound are primary drivers of sleep disruption and the first-night effect.

🔍️ Actionable insight: Use lighting and sound as tools to beat the first-night effect

Research suggest that light and sound are important when it comes to sleep disruption and the first-night effect. Building a consistent bedtime routine around these two factors can come in handy next time you find yourself travelling and sleeping away from home.

During hot summers I would often fall asleep to the drone of a fan. But as summer ended, I still needed the sound of the fan to fall asleep - it felt strange without!

🛠️ My current Sleepstack:

• Sleep mask: Light affects melatonin secretion even when your eyes are shut. I find that a good sleep mask is a game-changer when it comes to deeper sleep (and a great alternative to blackout curtains). I use this one at the moment - I don’t think I can go back to life without it.

• Screen filter software: Tools like f.lux (desktop), Twilight (Android) and Night Shift (iOS) change the colour of your display to based on the time of day to reduce overstimulating wavelengths.