Sleep Shakes Up Brain Cell Connections, Rewiring Memories

Zebrafish research reveals that during sleep, new brain cell connections are weakened, primarily in the first half of the night. This weakening allows new connections to be formed the next day. Sleep deprivation increases connections, but they are reduced when sleep is allowed. The findings suggest that sleep acts as a reset for the brain, supporting the Synaptic Homeostasis Hypothesis. However, the function of sleep in the latter half of the night remains unclear.

While we sleep, the brain weakens new connections between cells, or neurons, built when awake - but only during the first half of a night's sleep, a new research conducted on zebrafish has found.

Scientists explained that while awake, the connections between neurons get stronger and more complex, but if the activity continued without breaks, it would be ''energetically unsustainable.'' ''Too many active connections between brain cells could prevent new connections from being made the following day,'' said Jason Rihel from the University College London, UK, and the lead author of the study published in the journal 'Nature'.

''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,'' said Rihel.

For the study, the researchers monitored optically translucent zebrafish over several sleep-wake cycles. These animals possessed genes enabling the connections between neurons, or synapses, to be easily imaged.

The team found that the connections between brain cells were gained during waking hours and then lost during sleep. They also found that this depended on the sleep pressure, or the ''need for sleep'', that the animal built before being allowed to rest.

The researchers observed that when the fish were deprived of sleep for a few more hours, the connections between neurons continued to increase until the fish slept.

''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,'' said Rihel.

The rearrangement of connections happening in the first half of the animal's night sleep mirrors the pattern of slow-wave activity - part of sleep cycle strongest at the beginning of the night, the researchers said.

They also said that while the findings provide insights into the role of sleep, they leave an open question on its function during the latter half of the night's sleep.

The findings, however, support the theory that sleep acts as a 'reset' for the brain, or the Synaptic Homeostasis Hypothesis.

''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,'' said first author Anya Suppermpool from University College London.

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