The Penn campus at night

We all know that periods of sleep deprivation have an adverse effect on mental performance. Particularly when you’re young, you can tolerate an all-nighter or weeks of sleep deprivation, but it’s always a race against the clock to finish exams, papers, presentations (or a film shooting schedule, or delicate economic negotiations) before you collapse.


For a long time, researchers have known that sleep plays an important role in memory consolidation, and that during periods of little to no sleep, our ability to form memories and recall things on command degrades. Exactly why this is so, however, is something scientists are still working out. However, as the Guardian reports, a new study reveals one mechanism that’s responsible:

researchers show for the first time that sleep resets the steady build-up of connectivity in the human brain which takes place in our wakeful hours. The process appears to be crucial for our brains to remember and learn so we can adapt to the world around us.

The loss of a single night’s sleep was enough to block the brain’s natural reset mechanism, the scientists found. Deprived of rest, the brain’s neurons seemingly became over-connected and so muddled with electrical activity that new memories could not be properly laid down….

The results are a boost for what is called the synaptic homeostasis hypothesis of sleep, which was developed by scientists at the University of Wisconsin-Madison in 2003. It explains why our brains need to rest after a day spent absorbing all manner of information, from the morning news and the state of the weather, to a chat over lunch and what we must buy for tea.

Known more simply as SHY, the hypothesis states that when we are awake, the synapses that form connections between our brain cells strengthen more and more as we learn and eventually saturate our brains with information. The process requires a lot of energy, but sleep allows the brain to wind down its activity, consolidate our memories, and be ready to start again the next morning.

For those who can get to it, the original paper is in Nature Communications: “Sleep recalibrates homeostatic and associative synaptic plasticity in the human cortex.” Here’s the abstract:

Sleep is ubiquitous in animals and humans, but its function remains to be further determined. The synaptic homeostasis hypothesis of sleep–wake regulation proposes a homeostatic increase in net synaptic strength and cortical excitability along with decreased inducibility of associative synaptic long-term potentiation (LTP) due to saturation after sleep deprivation. Here we use electrophysiological, behavioural and molecular indices to non-invasively study net synaptic strength and LTP-like plasticity in humans after sleep and sleep deprivation. We demonstrate indices of increased net synaptic strength (TMS intensity to elicit a predefined amplitude of motor-evoked potential and EEG theta activity) and decreased LTP-like plasticity (paired associative stimulation induced change in motor-evoked potential and memory formation) after sleep deprivation. Changes in plasma BDNF are identified as a potential mechanism. Our study indicates that sleep recalibrates homeostatic and associative synaptic plasticity, believed to be the neural basis for adaptive behaviour, in humans.

In REST, I have a chapter on sleep as the original deliberate rest. Indeed, one of the most striking things I found with the people I studied was how many of them were really protective of naps and sleep: they seemed to be exquisitely aware of how their performance was affected by lost sleep.

Morning edits

Even the people who woke up super-early, and did some of their best work before they had fully shaken off drowsiness, didn’t scrimp on sleep: they went to bed earlier. Taking advantage of that more fluid mental state when you’re just awake and the world is still quiet is NOT AT ALL the same thing as staying up all night.