Most students are familiar with the dilemma created by a deadline to take a test when there are not enough hours left in the day; should I push through the exhaustion and study until I am finished, or should I sleep? While we might be tempted to choose the former, we are also familiar with the consequences of taking an exam after a night of little to no sleep. We wonder what happened to the information we were just reading five minutes before taking the test, which now seems impossible to recall. One possible answer is that interference of some sort did not allow for memory consolidation of this newly acquired information. Memory consolidation is defined as “the process by which experiences or information that has entered the memory system becomes strengthened so it is resistant to interference caused by trauma or other events” (Goldstein, 2011). So what does this have to do with sleep? Research has repeatedly shown that sleep can alter the consolidation of memories (Gais & Born, 2004). The specific type of memory we are most concerned with while taking an exam is called declarative memory, which involves the recollection of previously experienced events or facts (Goldstein, 2011). Because the mechanisms of memory acquisition, consolidation, and retrieval are quite complex, I will choose to focus on the relationship between the hippocampus, slow wave sleep, and declarative memory in our investigation into why lack of sleep might affect our memory during exams.
Although many structures are activated during memory consolidation, the hippocampus appears to be the most pronounced and most studied brain structure with regards to sleep and declarative memory (Marshall & Born, 2007). One proposed mechanism of declarative memory consolidation is believed to involve reactivation of newly acquired information in the hippocampal networks during slow wave sleep (SWS), which stimulates the transfer of these memory representations into neo-cortical networks (Gais & Born, 2004). One study that supports this theory involves neuroimaging in humans subsequent to learning a declarative task, which showed reactivation in the hippocampus during SWS (Marshall & Born, 2007). The amount of reactivation that was shown in the hippocampus was directly correlated with the participants’ performance on a recall task the following day. The increased activation of the hippocampus during SWS is part of the reason that researchers have continued to focus on the link between these stages of sleep and declarative memory.
Research has shown that declarative memory benefits specifically from an increase of slow wave sleep (Gais & Born, 2004). Slow wave sleep, characterized by slow delta waves, is often referred to as non-REM sleep stages 3 and 4, and dominates the first few hours of sleep we get each night (Freberg, 2010). One important study that showed the effects of stages of sleep and type of memory was performed by Plihal and Born (1997). The researchers assessed recall of paired-associative lists (declarative) and mirror-tracing skills (procedural) after two different retention intervals consisting of either early nocturnal sleep or late nocturnal sleep. They found that the participants’ recall of paired-associative lists improved more during the early sleep retention interval, which coincides with the time period that contains the bulk of our SWS (Plihal & Born, 1997). Recall of mirror-tracing skills improved more during the late sleep retention interval. Another study suggests that the decline in declarative memory recall that may occur during aging can be directly correlated with the decline of early nocturnal SWS during aging (Backhaus, et al., 2007). While slow wave sleep is not the only mechanism involved in improved consolidation and recall of declarative memories, it can certainly be claimed that it is a necessary component for optimal performance during an event that requires a high amount of recall of acquired facts, such as taking an exam.
The hippocampus-SWS link is an important component of memory consolidation with regards to declarative memory. Studies such as Plihal and Born (1997) have shown a direct link between improved declarative memory and SWS that occurs during early nocturnal sleep. In addition, hippocampal reactivation during SWS has been seen in neuroimaging studies, and greater activation was correlated with higher performance on recall tasks the following day (Marshall & Born, 2007). How does this apply to our original dilemma: stay up and study or get a good night’s rest before taking an exam? Based on the information that I have just described, I would say that early nocturnal sleep appears to be crucial to declarative memory consolidation, and consequently, recall during the exam. Perhaps this would mean that going to bed early the night before and waking up early to study would be a better solution than staying up late and getting only a few hours of sleep before a test. While more research would be needed to confirm this hypothesis, I certainly am going to consider this information when faced with another late night of studying.
References
Backhaus, J., Born, J., Hoeckesfeld, R., Fokuhl, S., Hohagen, F., & Junghanns, K. (2007). Midlife decline in declarative memory consolidation is correlated with a decline in slow wave sleep. Learning Memory, 14, 336-341. doi:10.1101/lm.470507
Freberg, L. A. (2010). Discovering Biological Psychology (2 ed.). (J. Potter, Ed.) San Luis Obispo, CA, USA: Wadsworth.
Gais, S., & Born, J. (2004). Declarative memory consolidation: mechanisms acting during human sleep. Learning Memory, 11, 679-685. doi:10.1101/lm.80504
Goldstein, E. B. (2011). Cognitive Psychology: Connecting Mind, Research, and Everyday Experience (3rd ed.). Belmont, CA: Wadsworth.
Marshall, L., & Born, J. (2007, November). The contribution of sleep to hippocampus dependent memory consolidation. Trends in Cognitive Science, 11(10), 442-450. doi:http://dx.doi.org/10.1016/j.tics.2007.09.001
Plihal, W., & Born, J. (1997). Effects of early and late nocturnal sleep on declarative and procedural memory. Journal of Cognitive Neuroscience, 9(4), 534-547. doi:10.1162/jocn.1997.9.4.534