Lack of sleep impairs the human brain's ability to store new information in memory, researchers have found. Past research has already shown that sleep is vital for consolidating recently-learned material but now Matthew Walker and colleagues have shown that sleep prior to learning is just as important.
“The implications of our findings have never been more relevant than in the present day, when sleep hygiene and total sleep time are declining across all age ranges”, the researchers said, pointing to the “all-nighter” before exams as the “quintessential example” of how we deliberately deprive ourselves of sleep.
The researchers scanned the brains of 28 participants who attempted to remember a series of pictures of people, landscapes, scenes and objects. Half the participants had slept the previous night as usual and acted as controls; the other participants had been kept awake, meaning they'd gone about 36 hours without sleep (the learning task was in the evening). Two days later, after everyone had two nights of normal sleep, the participants were shown more pictures and asked to identify those they'd been shown earlier in the week. Compared with the controls, the previously sleep deprived participants recognised 19 per cent fewer pictures.
Brain scans taken at the time the participants learned the pictures provided some clues as to how sleep deprivation had affected learning-related activity in the brain. Compared with the controls, the sleep deprived participants showed less activity in the hippocampus, the brain area associated with the laying down of episodic memories. Moreover, whereas the hippocampi of the controls was functionally connected with the frontal and parietal lobes, in the sleep deprived participants it was connected with basic alertness networks in the brain-stem - what the researchers said was “potentially a cooperative mechanism attempting to elevate levels of alertness during memory encoding”.
Why does lack of sleep have this effect on memory? The researchers aren't sure, but it's possible that lack of sleep denies the brain the opportunity to transfer episodic memories in the hippocampus into long term storage, so that the capacity of the hippocampus effectively becomes filled the longer we're awake.
Jyoo, S-S., Hu, P.T., Gujar, N., Jolesz, F.A. & Walker, M.P. (2007). A deficit in the ability to form new human memories without sleep. Nature Neuroscience, Advance Online Publication.
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