First half of sleep resets the brain — but what does the second half do?

LONDON — Humans have three basic needs for survival: food, air, and sleep. Having a good night’s rest is essential for the brain to organize itself, which includes either pruning newly learned information or transforming it into long-term memory. Interestingly, a new animal study reveals this resetting of brain connections only happens during the first half of your night’s sleep.

Sleep is one of the greatest mysteries of the human body. While we understand how sleep benefits health, the reasons behind certain processes remain elusive. The current study’s theory suggests that the brain may eliminate certain neural connections during sleep to prevent information overload, making it easier to learn new things the next day.

When a person is awake, the connections between brain cells grow stronger. However, creating a strong and complex neural connection requires more energy. If any brain connection could continue to grow stronger and more complex without limit, then it would become energetically unsustainable. According to the study published in Nature, the brain possibly evolved this resetting mechanism to prevent too many active brain connections, leaving little energy to forge new ones in the future.

“While the function of sleep remains mysterious, it may be serving as an ‘off-line’ period when those connections can be weakened across the brain, in preparation for us to learn new things the following day,” says Jason Rihel, a professor at the University College London’s Cell & Developmental Biology Department, in a media release.

However, the findings raise the question of why the brain only resets neural connections during the first half of the night and what function the second half of sleep holds.

Man sleeping, dreaming

The study used zebrafish, a common animal model for studying the brain during sleep. These Zebrafish were optically translucent, making it easier to record images of genes and structures that allow brain cells to communicate with each other. The study authors recorded brain activity when zebrafish were asleep and when they were awake.

Researchers found the brain added new connections when awake and lost a lot of them during the night. However, this depended on how much sleep pressure (the urge to sleep) the animal built up throughout the day before being allowed to rest. If scientists deprived the fish of sleep for a few extra hours, they noticed an increase in brain connections until the animal could sleep.

“If the patterns we observed hold true in humans, our findings suggest that this remodeling of synapses might be less effective during a mid-day nap, when sleep pressure is still low, rather than at night, when we really need the sleep,” adds Dr. Rihel.

The team notes that the rearranging of these connections between brain cells occurred during slow-wave sleep, a part of the sleep cycle that occurs during the beginning of the night.

“The findings add weight to the theory that sleep serves to dampen connections within the brain, preparing for more learning and new connections again the next day. But our study doesn’t tell us anything about what happens in the second half of the night,” says Anya Suppermpool, a professor at UCL Cell & Developmental Biology and the UCL Ear Institute who served as the lead study author.

Dr. Suppermpool notes the second half of the night might serve a completely different purpose. Instead of cleaning up brain connections, there are some theories suggesting the latter part of sleep is to remove toxins, dead cells, and other waste built up during the daytime. Additionally, another theory has looked into the connection between sleep and repairing damaged cells.

Leave a Reply

Your email address will not be published. Required fields are marked *