You check into a hotel. The bed is comfortable. The room is quiet. But you just cannot seem to get a good night’s sleep. Sound familiar?
It turns out your brain is not being difficult. It is actually doing something quite clever and scientists have a name for it.
Your Brain Does Not Always Sleep as One
Many birds, mammals, and yes, humans can sleep with one side of the brain in a deeper sleep while the other stays lighter and more alert. Researchers call this asymmetrical sleep.
In the most extreme version, one side of the brain can appear fully awake while the other is deeply asleep. Animals use this to keep functioning while resting. That is called unihemispheric sleep.
The First Night Effect
Even if your life has not exactly been full of danger, you have probably experienced a mild version of this. Researchers call it the first night effect, that restless first sleep in an unfamiliar place.
In sleep lab studies, participants showed deeper sleep in their right brain hemisphere and lighter sleep in their left during that first night. The lighter-sleeping left side stayed more active and responded more quickly to unexpected sounds.
Participants also woke up faster to those sounds on the first night than on later nights, once both hemispheres settled into deeper sleep. Researchers say this suggests asymmetrical sleep helps humans stay vigilant in unfamiliar surroundings.
Is Half-Sleep Actually Restful?
You might be wondering whether sleeping with only one side fully resting is good enough. Researchers are still working that out. But there is an interesting clue from fur seals.
Fur seals sleep unihemispherically at sea while migrating, partly to avoid predators. After a period of continuous stimulation, though, they showed a strong preference for sleeping with both hemispheres at once. That suggests full bilateral sleep may be more restorative.
So if you slept like a light bulb on that first night at your daughter’s house or in a new hotel room, do not be too hard on yourself. Your brain was simply keeping watch. It will settle down on night two.
