Can You Be Awake & Asleep At The Same Time?

There are two side to the story:

🎵 If you have half a brain 🎵

Many birds and mammals (humans included) can sleep with one brain hemisphere in deeper sleep while the other remains in lighter sleep and more active, allowing them to stay partially alert to their surroundings. This is called asymmetrical sleep.

In the most extreme form, one hemisphere can appear fully awake while the other is deeply asleep, allowing animals to maintain essential behaviors while resting. This is called unhemispheric sleep.

You may be wondering how restorative it is: it isn’t clear whether asymmetrical sleep provides all the same benefits as sleep involving both hemispheres fully, but fur seals (who are no strangers to unhemispheric sleep, due to sleeping at sea while migrating and also while having an aversion to being eaten by predators) showed a strong preference for bilateral sleep when recovering from continuous stimulation, suggesting it may be more restorative.

Even if your life hasn’t been fraught with danger, you might still have experienced a mild form of asymmetrical sleep, particularly during the first night in an unfamiliar environment, known as the “first night effect.”

For example, during the first night in a sleep laboratory, participants had deeper sleep in their right hemisphere and lighter sleep in their left, with the lighter-sleeping left hemisphere showing greater activity in response to infrequent sounds.

Further, participants woke and responded to infrequent sounds more quickly during the first night than during subsequent nights when both hemispheres experienced deeper sleep, suggesting that asymmetrical sleep indeed helps humans remain vigilant in unfamiliar environments.

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Take care!