A recent study conducted by UCL scientists found that during sleep, the brain weakens new connections between neurons that had been formed while awake, but only during the first half of a night’s sleep. The findings, published in Nature, support the Synaptic Homeostasis Hypothesis, which suggests that sleep acts as a reset for the brain. Lead author Professor Jason Rihel explained that during waking hours, connections between brain cells become stronger and more complex. If this activity were to continue without interruption, it would be energetically unsustainable and could prevent new connections from forming the next day.
The researchers used optically translucent zebrafish for the study, allowing them to easily image synapses, structures that communicate between brain cells. The team monitored the fish over several sleep-wake cycles and observed that brain cells gained connections during waking hours, which were then weakened during sleep. This phenomenon was found to be dependent on the amount of sleep pressure the animal had built up before resting. If the fish were deprived of sleep for longer periods, the connections continued to increase until the animal was allowed to sleep.
The study also revealed that the rearrangement of connections between neurons mostly occurred during the first half of a night’s sleep, corresponding to the pattern of slow-wave activity, a part of the sleep cycle that is strongest at the beginning of the night. This suggests that the brain may weaken connections within itself during sleep, preparing for new learning and connections the following day. However, the researchers noted that their study did not address what happens during the second half of the night. Other theories propose that sleep could serve as a time for the clearance of waste in the brain or repair for damaged cells.
Professor Rihel emphasized that if the observed patterns hold true in humans, the findings suggest that the remodeling of synapses might be less effective during a mid-day nap when sleep pressure is low compared to nighttime sleep when the need for sleep is higher. The research provides insight into the role of sleep in preparing the brain for learning and new connections. However, the exact function of the latter half of a night’s sleep remains unclear, leaving open questions around whether other processes, such as waste clearance or cellular repair, could be occurring during this time.
The study highlights the importance of sleep in maintaining the balance of connections between brain cells and preparing the brain for new learning opportunities. By weakening connections during the first half of the night’s sleep, the brain may be able to reset and optimize itself for the challenges of the following day. Further research is needed to explore the functions of the second half of a night’s sleep and to fully understand the complex relationship between sleep and brain function.
