Psychology student Hannah has a big exam coming up next week. Unfortunately, she has had little time to study for the exam. She realizes that she has underestimated the workload and she still has a lot of revision to do. The only solution she sees is to continue to study into the night. After a long week of late-night studying, the day of the exam has finally come. Hannah gets up in the morning and goes to her exam. She’s exhausted but it’s only two hours until she is done, and she can finally catch up on some much-needed sleep. She’s feeling positive as she enters the exam hall, after all she’s put a lot of work into her preparation – however, her spirits quickly change as she’s reading through the exam paper and drafting her answers. She’s having a lot of difficulty remembering the material she’s spent so much time studying on.

How is it possible that after all this time she’s invested into the material her memory of it is so poor?

The key skill that most exams test is recall of previously learned information, which is stored in our memory. So, to understand what happened to Hannah during her exam – let’s consider what sleep deprivation does to our memory systems. Long-term memory of facts or knowledge, also called declarative memory, is mostly stored in a brain region called the hippocampus. The hippocampus is a structure deep within our brain that has its name because it resembles the structure of a seahorse.

Neuroscience researchers decided to study this structure to see how it is affected by sleep deprivation. To study this, the researchers taught mice to perform a memory task in which the animal had to learn where an object was located in space (Havekes et al., 2016). They then sleep deprived these mice for one night and they let the animal perform the task again the next day. Mice are naturally curious animals, so once they are placed in the experimental chamber they will go and explore their environment looking for novel information. If the object is in the same place as the day before, they will spend little time exploring the object because they remember it from yesterday and it is not novel. However, if the object is placed in a different spot, this suddenly provides novelty to the mice, and they will spend more time exploring this new object.

Thus, by measuring the time spent exploring the objects, the memory of the mice can be tested. The researchers found that on the following day, animals who were sleep deprived showed much worse memory of where the object was located the previous day, compared to those mice that slept normally.

The researchers also explored more specifically how the hippocampi of these animals was affected by sleep deprivation. They specifically looked into a sub-structure of the hippocampus that is called CA1 and has been linked to spatial memory, which is what is needed to solve the object-based memory task (Tsien et al., 1996). Examining this CA1 area in depth, the researchers found that the number of synaptic connections were reduced in the mice that underwent sleep deprivation. Synaptic connections in our brain connect different neurons with each other. Neurons are the major cell type in our brain and they process the information that is presented to our brain. Neurons form connections with other neurons across the brain via their synapses. Thus, the number of synaptic connections is essential to the successful information processing of our brain. These findings therefore show us that getting less sleep directly influences the brain of mice and decreases their cognitive functions. This sounds very alarming as just one night of sleep deprivation can have such drastic effects on how our brain can process information. However, no reason to be alarmed just yet.

The researchers did find that once the mice were allowed to catch up on some sleep, their performance levels as well as brain structure recovered to previous functionality (Havekes et al., 2016).

Similar associations between sleep deprivation and performance on cognitive tasks can be seen in humans. Humans do have higher cognitive functions compared to mice, and therefore it can be more difficult to unravel which specific cognitive functions are affected by sleep deprivation.

The general consensus is that sleep deprivation does affect cognitive performance in humans (Killgore, 2010).

It appears that simple tasks, such as simple decision-making are not affected. However, when it comes to more complex cognitive tasks, e.g., problem solving tasks that require innovative or creative solutions, individuals who did not sleep well the night before perform significantly worse compared to well-rested individuals. Thus, in the future, if you find yourself in a situation similar to Hannah you might want to keep in mind what you read in this blogpost and make getting a good night’s sleep a priority. It certainly can’t hurt.

References

Havekes, R., Park, A. J., Tudor, J. C., Luczak, V. G., Hansen, R. T., Ferri, S. L., Bruinenberg, V. M., Poplawski, S. G., Day, J. P., Aton, S. J., Radwańska, K., Meerlo, P., Houslay, M. D., Baillie, G. S., & Abel, T. (2016). Sleep deprivation causes memory deficits by negatively impacting neuronal connectivity in hippocampal area CA1. ELife, 5, e13424. https://doi.org/10.7554/eLife.13424

Killgore, W. D. S. (2010). Effects of sleep deprivation on cognition. In G. A. Kerkhof & H. P. A. van Dongen (Eds.), Progress in Brain Research (Vol. 185, pp. 105–129). Elsevier. https://doi.org/10.1016/B978-0-444-53702-7.00007-5

Tsien, J. Z., Huerta, P. T., & Tonegawa, S. (1996). The essential role of hippocampal CA1 NMDA receptor–dependent synaptic plasticity in spatial memory. Cell, 87(7), 1327–1338.

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