Consciousness and unconsciousness

Many animals have developed unique ways to sleep that help them stay alert while still getting the rest they need. One fascinating adaptation is asymmetrical sleep, where different parts of the brain rest at different times, allowing them to manage both sleep and awareness simultaneously.

Take bottlenose dolphins, for example. Dolphins face the challenge of needing to swim and breathe consciously to avoid drowning. When they have a newborn calf, they must swim almost continuously for weeks to keep it safe. To manage this, dolphins use a special form of sleep called unihemispheric sleep. In this mode, only one hemisphere of their brain sleeps at a time while the other remains active. This way, they can keep swimming and breathing while still getting some rest. By alternating which hemisphere is awake, dolphins can ensure they stay alert and continue their essential behaviors without interruption.

Fur seals, which migrate long distances at sea, use a similar strategy. During their journeys, fur seals float horizontally in the water, keeping one eye open and one nostril above the surface to breathe while closing the eye and nostril on the other side. This allows them to stay aware of potential predators and environmental changes while still getting some sleep. Research has shown that after periods of constant alertness, fur seals prefer to sleep with both hemispheres at rest, which seems to be more restorative for them.

Birds also use various forms of asymmetrical sleep. For instance, mallard ducks often sleep in groups, with some ducks on the edges of the group needing to stay more alert. These ducks might use unihemispheric sleep, keeping one eye open and the corresponding brain hemisphere active to watch for potential threats. Similarly, frigatebirds, which fly long distances across oceans, can sleep in short bursts with one or both hemispheres at a time. This method allows them to rest while riding air currents, although they still sleep less than 8% of what they would on land due to their constant need to stay alert.

Humans also experience a subtle form of asymmetrical sleep, especially when in new or unfamiliar environments. Scientists have long observed that people often sleep poorly their first night in a new place, and this night’s data is usually discarded in sleep studies. In 2016, researchers discovered that this "first night effect" is actually a mild form of asymmetrical sleep in humans. During the first night in a new environment, people tend to have deeper sleep in the right hemisphere of their brain and lighter sleep in the left. This lighter sleep in the left hemisphere helps them stay alert to unfamiliar sounds, allowing them to respond more quickly to potential disturbances. This suggests that, like other animals, humans use this form of sleep to stay vigilant when they are in new or potentially risky situations.

In summary, asymmetrical sleep is a fascinating adaptation that helps various animals balance the need for rest with the need to stay alert. Whether through unihemispheric sleep or maintaining partial wakefulness, these strategies allow animals and humans alike to navigate their environments safely and effectively while managing their sleep needs.