Sleep is ubiquitous in animals, but its specific function is unknown. It is now believed that the main function of sleep is to allow individual neurons to perform prophylactic cellular maintenance. Just as muscle cells must rest after strenuous exercise to prevent long-term damage, brain cells must also rest after strenuous synaptic activity. Periods of reduced synaptic input, i.e., off periods or down states, are necessary for this maintenance, which in turn requires a globally synchronized state of neuronal activity that reduces sensory input and immobility – well-known sleep manifestations.
Sleep is an essential act for survival, and the duration and quality of sleep can affect the mental state of the next day, and the most intuitive response is napping, slow thinking and decreased reflexes. In addition to these "side effects" that can be felt, the brain is also very sensitive to lack of sleep.
We may think that the body is completely resting when we sleep, but in fact there are still complex activities in the brain that help it recover from the heavy work of the day, such as the lymphoid system, which flushes clean cerebrospinal fluid into the brain, and then takes away the waste accumulated during the day, reducing the damage to nerve cells caused by toxic substances.
Image source: 123RF
In a recent study in the journal Nature, scientists from University College London found that sleep also plays an important role, that is, to help the brain "reset". When we are awake, the brain needs to control and coordinate various input signals and give instructions for action, which means that many neurons will have complex connections to transmit information.
These connections are not without a price, and they tend to consume a lot of energy. According to the synaptic homeostasis hypothesis, synaptic increase during waking cannot occur indefinitely, and if it is maintained all the time, it is not only not conducive to learning new things, but also unsustainable in terms of energy supply. Sleep is the key stage in reducing and disconnecting synapses, and this change is present for the vast majority of excitatory synapses in the brain.
To get a clearer view of neuronal activity during sleep, the authors chose transparent zebrafish as a model. They selectively labeled zebrafish synapses by genetic engineering, making them easier to image, and observed changes in synaptic activity over a 24-hour cycle.
The study regulates the circadian rhythm of zebrafish by turning the lights on and off, and because zebrafish are diurnal, they are more active when they are fully lit. The imaging results show that the number of synapses gradually increases during the day and decreases at night, falling back to lower levels. According to the authors' neuronal typing of zebrafish, the study-defined synapses for type 2 neurons increased by an average of 15.3 during the day and decreased by 17.7 at night, and type 4 neurons had an average increase of 8.5 synapses during the day and a decrease of 8.2 at night. This change results in a stable, balanced state of the synapse.
▲Synapses decrease when you sleep at night (Image source: Reference [1])
In addition to this, the study also found two key characteristics, the first is that whether this synaptic homeostasis process occurs depends on how much sleep stress is, in layman's terms, how long you haven't slept. The greater an individual's need for sleep, the more synaptic clearance events will occur during sleep. On the other hand, if the need for sleep is not high, the number of synapses in zebrafish will slowly increase rather than decrease, even if the sleep state is induced by drugs.
"If this pattern can be confirmed in humans, it means that a short break from the lunch break does not have much effect on synaptic balance, and it is important to get a good night's sleep at night when the need for sleep is high." Professor Jason Rihe, corresponding author of the study, said. Another interesting finding was that the authors only observed synaptic clearance in the first half of the nocturnal sleep phase of zebrafish, while there was no change in the second half.
The authors note that this synaptic clearance helps generate new synaptic connections the next day, preparing them to learn new things. There are other effects, such as the repair of cell damage may be done in the second half of sleep, which requires more research in the future to solve the mystery.
Resources:
[1] Jason Rihel, Sleep pressure modulates single-neuron synapse number in zebrafish, Nature (2024). DOI: 10.1038/s41586-024-07367-3
[2] Scientists say sleep resets brain connections—but only for first few hours. Retrieved May 7, 2024 from https://medicalxpress.com/news/2024-05-scientists-resets-brain-hours.html