Longevity Briefs: Can Sleep Loss Be Compensated With 'Fake Sleep'

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Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

It is clear that sleep provides a vital function – evolution wouldn’t have favoured a process that makes us unconscious and paralyses us for a third of our lives if it wasn’t very important. But what exactly is the brain doing while we are asleep? Sleep is divided into two broad categories: REM (rapid eye-movement) sleep, in which dreams occur, and NREM (non-REM) sleep. These phases alternate throughout the night as part of 90 minute cycles. The most important phase of sleep appears to be the deepest phase of NREM sleep, also known as slow wave activity (SWA) or delta wave sleep. As its name implies, SWA involves slow waves of electrical activity as many neurons all fire in unison in a slow on-off pattern. This is thought to be important in learning and memory, as it results in a stronger signal that can synchronise different areas of the brain. Think of a stadium full of people trying to make themselves understood at great distance – it is better if everyone shouts slowly and together.

The importance of these slow waves themselves has always been suspected, but never truly tested. While we know that the SWA stage of sleep is important, the waves themselves could have been a side effect of whatever important process was going on beneath the surface. This study, however, provides evidence that these slow waves are important. Not only that, but it also suggests that artificially inducing slow waves without sleep can replicate some of the benefits of sleep.

The discovery:

To test whether slow waves themselves were important, researchers designed two genetically engineered mouse models in which SWA could be artificially induced. These models were slightly different, but both worked in broadly the same way: genes encoding light-sensitive channels were introduced into a specific population of neurons. This allowed researchers to use light, delivered via an implanted optic fibre probe, to inhibit the activity of those neurons at will. By delivering light to those neurons rhythmically, they could then artificially induce SWA patterns.

With the mouse models established, researchers then investigated whether artificial SWA activity could lessen the need for sleep. Mice were sleep deprived for 5 hours, but during the last 30 minutes, received artificially induced SWA in one brain hemisphere. Once the mice actually went to sleep, brain recordings showed that SWA was reduced in the hemisphere that received artificially induced SWA compared to the other hemisphere. This is usually an indication of reduced ‘sleep pressure’. In other words, it appeared that the artificial SWA served as a substitute for normal SWA, meaning that hemisphere needed less of it during actual sleep.

This experiment suggested that the brain was indeed responding to the need for slow wave activity itself, but it still didn’t establish a link between SWA and brain function. To test this, researchers divided mice into three groups of roughly 10 mice each. Each group was given a memory test that involves their ability to find an object based on the texture of the floor as a cue. After the learning period, one group was allowed to sleep normally while the other two were sleep deprived for one hour. However, one of the sleep deprived groups received artificial SWA during the sleep deprivation phase. When researchers tested the mice’s recall ability 24 hours later, they found that the sleep deprived group performed significantly worse than the sleep group as expected, but there was no statistically significant difference in learning between the group that slept normally and the group that received artificial SWA during sleep deprivation.

This graph shows the proportion of time spent by mice in the ‘correct’ area with the floor texture, in the sleep group (red), sleep deprived group (SD, grey) and sleep deprived SWA group (SD + off induction, blue).
The boxes represent the performance of the middle 50% of mice, while the solid horizontal line represents the median (the performance exceeded by half of the mice in that group. The dashed horizontal line represents the mean (average), and the vertical lines extending above and below the boxes show the best and worst performances after extreme outliers were excluded.
* at the top of the graph denotes statistically significant differences between groups.
*Induction of cortical on/off periods in awake mice fulfills sleep functions*

The implications:

This research suggests that SWA is important for learning and memory consolidation during sleep, and that artificially stimulating SWA can be a substitute for NREM sleep, at least to some extent, preventing impaired learning and memory. Researchers noted that overall synaptic strength was weaker in the hemisphere that received artificial SWA. They hypothesise that a function of SWA is actually to globally weaken synapses that have become stronger during the day, thereby preserving learning capacity.

One interesting question raised by this is whether artificially inducing SWA in humans would be beneficial. There has long been the idea that listening to binaural delta waves (mismatched audio frequencies delivered to each ear such that they overlap to create a delta wave frequency) can improve learning and memory. The human data for this is quite limited, but the present study provides a potential mechanism for how this could work. On the other hand, for a person getting adequate sleep, the brain might simply compensate by spending less time in NREM sleep, as occurred in mice in this study.