What happens in the brain during the first hours of sleep at night?

During sleep, the brain weakens new connections between neurons that were formed while awake, but only during the first half of the night's sleep, according to a new study.

The UCLA scientists say their findings, published in the journal Nature, provide insight into the role of sleep, but still leave open the question of what function the latter half of a night's sleep performs.

The team points out that the study supports the Synaptic Homeostasis Hypothesis hypothesis, a major theory about the purpose of sleep that proposes that sleep acts as an important reset mechanism for the brain, ensuring energy sustainability and readiness for learning.

Lead author Professor Jason Ryhill, from the Department of Cell & Developmental Biology at UCL Cell & Developmental Biology, said: “When we are awake, the connections between brain cells become stronger and more complex. If this activity continues unabated, it will be energetically unsustainable. “Having too many active connections between brain cells can prevent new connections from being created the next day.”

He continued: "Although the function of sleep is still mysterious, it may serve as a period of disconnection, as those connections in all parts of the brain can be weakened, in preparation for us learning new things the next day."

In this study, the scientists used optically transparent zebrafish with genes that enable synapses (the structures that communicate between brain cells) to be easily imaged. The research team monitored the fish over several cycles of sleep and wakefulness.

They found that brain cells gain more connections during waking hours, then lose them during sleep. They found that this depends on how much sleep pressure (need for sleep) the animal has built up before being allowed to rest, and when sleep is postponed, sleep pressure increases the decline in neural connections.

The team found that the rearrangement of connections between neurons occurred mostly in the first half of the animal's nighttime sleep. This reflects a pattern of slow wave activity (commonly referred to as deep sleep), which is part of the sleep cycle that is strongest at the beginning of the night.

First author Dr Anya Sobermbol, from the Department of Cell and Developmental Biology and the Ear Institute of the School of Brain Sciences at University College London, said: “Our findings add weight to the theory that sleep acts to weaken connections within the brain, preparing for further 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. There are other theories about sleep being a time to remove waste from the brain, or repair damaged cells, and perhaps other functions begin in the second half of the night."