During the first half of sleep, the brain actively weakens the synaptic connections formed during wakefulness, according to a new study conducted by scientists at UCL. This finding provides support for the Synaptic Homeostasis Hypothesis, which suggests that sleep serves as a crucial reset that prepares the brain for new learning by reducing synaptic strength.

The researchers used optically translucent zebrafish for the study, as their genes enabled synapses (structures that communicate between brain cells) to be easily imaged. They monitored the fish over several sleep-wake cycles to observe the changes in synaptic connections.

The study demonstrated that the brain reduces synaptic connections primarily during the first half of sleep, indicating a reset mechanism that prepares the brain for new learning the next day. Lead author Professor Jason Rihel explained that during wakefulness, the connections between brain cells become stronger and more complex. If this activity were to continue without interruption, it would be energetically unsustainable and hinder the formation of new connections.

The extent of synaptic weakening was found to be dependent on the sleep pressure accumulated, with higher sleep pressure leading to more significant synaptic reduction. This implies that shorter naps during the day, when sleep pressure is lower, may not offer the same benefits as a full night of sleep in terms of synaptic weakening.

Interestingly, the researchers found that the rearrangements of connections between neurons mostly occurred in the first half of the animal's nightly sleep, aligning with the peak of slow-wave activity. However, the purpose of the latter half of sleep remains less understood, leaving an open question regarding its function.

These findings provide valuable insights into the role of sleep in synaptic modulation, which could have implications for understanding human sleep and its essential functions in brain health. Professor Rihel suggests that sleep may serve as an "off-line" period during which connections can be weakened across the brain, allowing for the consolidation of new information and learning the following day.

While the study was conducted on zebrafish, the researchers believe that the observed patterns may hold true in humans as well. However, further research is needed to validate this hypothesis and uncover the specific functions of the second half of the night's sleep.

Overall, this study contributes to our understanding of the intricate processes that occur during sleep and highlights the importance of a good night's rest for optimal brain function and learning.