Can You Be Awake and Asleep at the Same Time? Exploring Asymmetrical Sleep

Have you ever wondered how some animals manage to sleep while staying alert to danger? The answer lies in a fascinating phenomenon called asymmetrical sleep. This is where parts of the brain are asleep while other areas remain active. It's not just for animals; humans experience it too!

The Basics of Asymmetrical Sleep

All vertebrate brains have two hemispheres, the right and left. Typically, brain activity is similar across both during sleep. However, during asymmetrical sleep, one hemisphere can be in deep sleep while the other is in a lighter state. In an extreme version, known as unihemispheric sleep, one hemisphere appears completely awake while the other is in deep sleep.

Examples in the Animal Kingdom

Several animals rely on asymmetrical sleep for survival:

• Bottlenose Dolphins: These marine mammals consciously control their breathing and must surface for air. When they have a newborn calf, they swim nonstop for weeks to keep it safe. Dolphins use unihemispheric sleep, allowing them to continue swimming and breathing while resting.
• Fur Seals: These animals migrate at sea for weeks. They use unihemispheric sleep while floating horizontally, keeping one eye open to stay alert to threats.
• Mallard Ducks: Ducks sleeping in groups on the peripheries use unihemispheric sleep, keeping their outward-facing eye open and the corresponding brain hemisphere more active.
• Frigatebirds: These birds undertake non-stop transoceanic flights and sleep in seconds-long bursts with one or both hemispheres at a time.

Benefits of Asymmetrical Sleep

It's unclear if asymmetrical sleep provides the same benefits as sleep in both hemispheres. Fur seals, when constantly stimulated, rely on asymmetrical sleep. However, during recovery, they prefer sleep across both hemispheres, suggesting it's more restorative. Dolphins can maintain high levels of alertness for at least five days by switching which hemisphere is awake, getting several hours of deep sleep in each hemisphere throughout a 24-hour period.

Asymmetrical Sleep in Humans

Ever felt groggy after your first night in a new place? That's because part of your brain might've been only somewhat asleep. Scientists discovered that the "first night effect" is a subtle version of asymmetrical sleep in humans.

During the first night in a new environment, people experience deeper sleep in their right hemisphere and lighter sleep in their left. The lighter-sleeping left hemisphere shows greater activity when exposed to infrequent sounds. This suggests that humans use asymmetrical sleep for vigilance in unfamiliar environments.

The "First Night Effect"

Scientists have long recognized that people sleep poorly their first night in a lab. In fact, it's common practice to discard the data from that night. Research has shown that during this "first night effect," the brain exhibits deeper sleep in the right hemisphere and lighter sleep in the left hemisphere.

Vigilance in Unfamiliar Environments

When exposed to infrequent sounds, the lighter-sleeping left hemisphere shows greater activity. Participants also wake up and respond to these sounds faster during the first night compared to nights with deep sleep in both hemispheres. This indicates that humans, like other animals, use asymmetrical sleep for vigilance, especially in new surroundings.

Conclusion

While your hotel room isn't trying to eat you, your brain is still keeping you alert, just in case. Asymmetrical sleep is a fascinating adaptation that allows animals, including humans, to balance the need for rest with the need for vigilance. It highlights the incredible adaptability of the brain and its ability to protect us even in our most vulnerable state.