One of the ways to look at a biological function of sleep is to see what happens when you don’t sleep. All of us have experienced some level of sleep deprivation during exam time, flight travel across global time zones etc. This is usually manifested as irritability, lack of focus, mood swings and therefore affects the quality of our daily life. As you might have noticed, the larger the sleep debt, larger and deeper is the compensatory sleep. This is referred to as “sleep homeostasis”—so one cannot really adapt to lower than required sleep amounts. This is true in all species studied so far, from fruit flies to humans. Animal studies have shown that long term sleep deprivation is fatal. From an animal point of view, it almost feels counter-productive for an animal to sleep and “disconnect” with the environment at the cost of not engaging in other activities and more importantly, making themselves vulnerable to predators. Therefore, sleep must serve a higher universal function to offset this risk. The simple yet elegant hypothesis called Synaptic Homeostasis Hypothesis (SHY) attempts to explain this essential function of sleep (1, 2).
What happens during waking? The brain, as we know is extraordinarily plastic with number and efficacy of synapses always in flux. During waking, we interact with the environment and acquire information. Due to the high noradrenaline content in the brain during waking, this is manifested in our brain as long term potentiation (LTP) and an overall increase in synaptic strength. This feature enables us to learn new facts, make new memories and adapt to our surroundings. We absorb a huge amount of information during waking, leading to changes in synaptic strength and making new connections. Also for important pieces of information to percolate into other neuronal groups deep inside the brain, connections have to be strengthened. So, increase in net synaptic strength is a hallmark of wakefulness. Moreover, biochemical markers and dendritic morphological studies provide evidence in support of this hypothesis.
Now, let us consider a scenario where this process goes on for a long time. One can almost imagine it to be like an overgrown rose bush. But this would take a toll on space and energy resources making this process unsustainable for a long time. Also, if all the connections are strengthened, neurons become increasingly excitable and this leads to a high “noise” in the brain making it difficult to identify the true signal thereby impairing further learning process. Therefore, what is needed is a regulation of synapses to return to a sustainable level so that the energy costs, signal-to-noise ratio and learning capacity is returned to baseline level.
With all the input coming in continuously during waking, it does become difficult for the brain to regulate synaptic strength. However, during sleep, there is no external input and therefore, the time is perfect for the brain to sample neuronal inputs “off-line” and determine which synapses should remain and which should be pruned. This is a homeostatic process because all the synapses are downscaled to a similar range, while maintaining the relative strength of the synapses. The brain neurons undergo localized or large-scale SWA or slow-wave activity wherein the synchronous wave activity leads to an overall decrease in synaptic strength or synaptic scaling. Downscaling during sleep improves signal to noise ratio allowing for memory consolidation and also for new learning to occur during subsequent waking.
So, sleep well today so that your brain is ready to learn tomorrow. To quote the authors of the synaptic homeostasis hypothesis- sleep is “the price we pay for plasticity”(3)
- Tononi, G. and Cirelli, C. (2012) Time to Be SHY? Some Comments on Sleep and Synaptic Homeostasis. Neural Plast. 2012, 415250.
- Frank, M.G. (2012) Erasing synapses in sleep: is it time to be SHY? Neural Plast. 2012, 264378.
- Tononi, G. and Cirelli, C. (2012) Sleep function and synaptic homeostasis. Sleep Med Rev. 10, 49–62.
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