Sleep’s First Half Crucial for Brain Reset

Summary: During the first half of sleep, the brain actively weakens the synaptic connections formed during wakefulness, supporting the Synaptic Homeostasis Hypothesis on the purpose of sleep. This finding suggests that sleep serves as a crucial reset that prepares the brain for new learning by reducing synaptic strength.

The researchers observed that synaptic weakening mostly occurs in the first half of the night, aligning with the peak of slow-wave activity, while the function of the latter half remains less understood. This insight into sleep’s role in synaptic modulation could have implications for understanding human sleep and its essential functions in brain health.

Key Facts:

  1. Synaptic Weakening During Sleep: The study demonstrates that the brain reduces synaptic connections primarily during the first half of sleep, suggesting a reset mechanism that prepares for new learning the next day.
  2. Role of Sleep Pressure: The extent of synaptic weakening is dependent on the sleep pressure accumulated; higher sleep pressure leads to more significant synaptic reduction.
  3. Implications for Napping: The findings suggest that shorter naps during the day, when sleep pressure is lower, may not offer the same synaptic weakening benefits as nocturnal sleep.

Source: UCL

During sleep, the brain weakens the new connections between neurons that had been forged while awake – but only during the first half of a night’s sleep, according to a new study in fish by UCL scientists.

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.

This shows a woman sleeping.
For the study, the scientists used optically translucent zebrafish, with genes enabling synapses (structures that communicate between brain cells) to be easily imaged. Credit: Neuroscience News

The researchers say the study supports the Synaptic Homeostasis Hypothesis, a key theory on the purpose of sleep which proposes that sleeping acts as a reset for the brain.

Lead author Professor Jason Rihel (UCL Cell & Developmental Biology) said: “When we are awake, the connections between brain cells get stronger and more complex. If this activity were to continue unabated, it would be energetically unsustainable. Too many active connections between brain cells could prevent new connections from being made the following 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.

Professor Rihel added: “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.”

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.

First author Dr Anya Suppermpool (UCL Cell & Developmental Biology and UCL Ear Institute) said: “Our 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 our study doesn’t tell us 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.”

About this sleep and neuroscience research news

Author: Chris Lane
Source: UCL
Contact: Chris Lane – UCL
Image: The image is credited to Neuroscience News

Original Research: Open access.
Sleep pressure modulates single-neuron synapse number in zebrafish” by Jason Rihel et al. Nature


Sleep pressure modulates single-neuron synapse number in zebrafish

Sleep is a nearly universal behaviour with unclear functions. The synaptic homeostasis hypothesis proposes that sleep is required to renormalize the increases in synaptic number and strength that occur during wakefulness.

Some studies examining either large neuronal populations or small patches of dendrites have found evidence consistent with the synaptic homeostasis hypothesis, but whether sleep merely functions as a permissive state or actively promotes synaptic downregulation at the scale of whole neurons is unclear.

Here, by repeatedly imaging all excitatory synapses on single neurons across sleep–wake states of zebrafish larvae, we show that synapses are gained during periods of wake (either spontaneous or forced) and lost during sleep in a neuron-subtype-dependent manner.

However, synapse loss is greatest during sleep associated with high sleep pressure after prolonged wakefulness, and lowest in the latter half of an undisrupted night.

Conversely, sleep induced pharmacologically during periods of low sleep pressure is insufficient to trigger synapse loss unless adenosine levels are boosted while noradrenergic tone is inhibited.

We conclude that sleep-dependent synapse loss is regulated by sleep pressure at the level of the single neuron and that not all sleep periods are equally capable of fulfilling the functions of synaptic homeostasis.

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