Sleeping early crucial for brain reset, says expert

Researchers at the University of California have conducted a new study on zebrafish that explores the mysterious realm of sleep. Their research, which was published in Nature, clarifies the brain’s activity when we sleep, but it raises some concerns about what happens in the latter portion of the night.

The hypothesis that sleep serves as a mental “reset” is supported by the studies. The brain weakens recently created neuronal connections when you sleep (a process known as synaptic pruning). The first half of sleep appears to be when this trimming occurs most frequently.

The second half’s goal is still unknown, despite the first half’s emphasis on simplifying connections. Scholars speculate that it may entail additional procedures such as eliminating trash or repairing cells. The new research sheds light on the mysterious world of sleep, but also leaves some questions swimming. The researchers say their findings, published in Nature, provide insight into the role of sleep, but still leave an open question around what function the latter half of a night’s sleep serves.

The Synaptic Homeostasis Hypothesis, a major theory on the function of sleep that suggests that sleep serves as a reset for the brain, is supported by the results of this study.

“The connections between brain cells get stronger and more complex when we are awake,” stated lead author Professor Jason Rihel of UCL Cell & Developmental Biology. This action would become energy unsustainable if it kept up. An excessive number of active connections among brain cells may inhibit the formation of new connections the next day.

“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.”

For the study, the scientists used optically translucent zebrafish, with genes enabling synapses (structures that communicate between brain cells) to be easily imaged. The research team monitored the fish over several sleep-wake cycles.

The researchers found that brain cells gain more connections during waking hours, and then lose them during sleep. They found that this was dependent on how much sleep pressure (need for sleep) the animal had built up before being allowed to rest; if the scientists deprived the fish from sleeping for a few extra hours, the connections continued to increase until the animal was able to sleep.

“If the patterns we observed hold true in humans, our findings suggest that this remodelling 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,” Rihel added.

The researchers also found that these rearrangements of connections between neurons mostly happened in the first half of the animal’s nightly sleep. This mirrors the pattern of slow-wave activity, which is part of the sleep cycle that is strongest at 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 the study doesn’t tell anything about what happens in the second half of the night. There are other theories around sleep being a time for clearance of waste in the brain, or repair for damaged cells — perhaps other functions kick in for the second half of the night.