Faculty peer reviewed by Dr. David Rapoport
The role of sleep in memory formation is an intriguing topic that has garnered widespread interest among researchers in recent years. The subject has seen a doubling in the number of publications every decade, yet the mechanism by which memories are formed remains elusive. There is little debate that sleep is important for memory, which begs the question: What does the role of sleep in memory imply for the infamously sleep-deprived medical trainee? This brief review will cover some basic concepts about sleep and their potential application to the problem of sleep deprivation in medical training.
Memory systems can be divided into two forms: declarative and non-declarative.1 The former encompasses episodic and semantic memory (such as steps in the Krebs cycle) while the latter includes non-conscious learning such as conditioning, priming, and procedural skill (such as how to tie a knot). Primate studies have implicated structures in the medial temporal lobe as critical components for declarative memory.2 This neural system consists of the hippocampus and the surrounding entorhinal, perirhinal, and parahippocampal cortices. These structures are involved in the developing stages of memory, a series of steps by which we learn. Simplistically, these stages are encoding (acquisition of a new lesson learned), consolidation, retention, and recall. Recent findings have extended how we think about consolidation: memories are not just stabilized in a corner of our cortex, but rather are enhanced and reconsolidated as they are reactivated over periods of wakefulness and sleep.3
Sleep is important for all stages of memory formation, suggesting that adequate sleep before encoding may play an important role in consolidation, just as does sleep after learning a task. A study done in the UK showed that subjects who were deprived of sleep before cognitive tasks performed much worse on testing than controls, despite caffeine consumption.4 (Interestingly, the sleep deprivation group also had poor insight into their poor performance.) Post-training sleep is of paramount importance also. On declarative memory tasks, subjects deprived of sleep for 36 hours after learning new material exhibited a 40% reduction in memory retention compared to those who slept.5 Both rapid eye movement (REM) and non-REM, or slow wave, sleep are thought to be involved in memory consolidation. Neuroimaging studies show that “reactivation” during REM sleep and reemergence of hippocampal activation during slow wave sleep directly correlate with task improvement the next day.6,7 So, what does this mean for those who do not get adequate sleep?
Medical students and residents lie at an interesting intersection of sleep and memory. On the one hand, they are asked to memorize vast amounts of information under the fear that any misstep can prove fatal. On the other hand, they are sometimes required to work long hours that may include 24-to-36-hour calls. To test whether medical trainees are affected by such working hours, researchers administered cognitive tests to OB/GYN house staff and students at Emory University before and after on-call duty. There was a statistically significant decrease in test scores after an on-call period, suggesting a decline in cognitive functioning towards the end of a shift.8
In addition to acute sleep deprivation in the on-call setting, chronic sleep deprivation has implications on memory and cognitive function. In a comparison of subjects who were restricted to 8, 6, or 4 hours of sleep over a 14-day period to those who were subject to 2 days of sleep deprivation with normal sleep before and after, researchers found that consistent restriction of sleep to 6 hours per night or less caused cognitive deficits as severe as those produced by 2 nights of acute sleep deprivation.9 However, chronically sleep-deprived subjects were unaware of their increasing cognitive deficits, explaining the assumption that chronic mild sleep deprivation is not deleterious. So, when not on-call, do students sleep enough? Not surprisingly, the answer is no. Students in a quality-of-life survey administered before, during, and after the notoriously difficult surgery clerkship reported a mean sleep duration of 6.3 hours per night at the beginning which declined to 5.7 hours of sleep each night three quarters of the way through the clerkship.10 Scores on quality-of-life scales declined, while scores on depression scales increased during the clerkship period. In essence, students fail to follow professors’ advice to “Get a good night’s sleep” before any test of cognitive performance.
What does this all imply for those in medical training? Students should be aware of both the benefits of sleep on memory and the effects of sleep deprivation on both cognitive and physical performance. Rather than attempt to overdo the clerkship time requirements or stay up all night “cramming,” students would profit by taking steps to protect sleep time. Furthermore, mentors should be aware of sleep deprivation problems and become advocates for protected sleep time. Ideal use of off time includes getting quality sleep that would facilitate an optimal cognitive environment for memory encoding, consolidation, and recall.
Megan Mulligan is a 4th year medical student at NYU School of Medicine.
Commentary by Dr. David Rapoport, Associate Professor of Medicine
Sleep, while essential for memory and task performance, does compete with waking time. Thus, both medical students and residents need to acquire skills or knowledge (medical facts, number of surgical cases) to become better trained, and this is not always done without sacrificing some sleep time, as the total number of hours/days/years is limited. The problem is the tradeoff. This is essential to keep in mind so as not to fall into the trap I perceive in the Accreditation Council for Graduate Medical Education’s position that all sleep restriction is bad. Speaking teleologically, I would argue that, while optimal performance on memory and task may occur after optimal sleep, we are obviously designed to tolerate some sleep deprivation at a modest cost in performance (and memory), and sometimes this pays off handsomely. The problem is when we try to push this too far, as is traditionally done in both medical training and in many high-power stressful jobs.
1. Squire LR, Zola SM. Structure and function of declarative and nondeclarative memory systems. Proc Natl Acad Sci USA. 1996;93(24):13515-13522.
2. Squire LF, Zola-Morgan S. The medial temporal lobe memory system. Science. 1991;253(5026):1380-1386.
3. Walker MP, Brakefield T, Hobson JA, Stickgold R. Dissociable stages of human memory consolidation and reconsolidation. Nature. 2003;425(6958):616-620.
4. Harrison Y, Horne JA. Sleep loss and temporal memory. Q J Exp Psychol A. 2000;53(1):271-279.
5. Walker MP, Stickgold R. Sleep, memory, and plasticity. Annu Rev Psychol. 2006;10:139-166.
6. Maquet P, Laureys S, Peigneux P et al. Experience-dependent changes in cerebral activation during human REM sleep. Nat Neurosci 2000;3(8):831-836.
7. Peigneux P, Laureys S, Fuchs S, et al. Are spatial memories strengthened in the human hippocampus during slow wave sleep? Neuron. 2004;44(3):535-545.
8. Halbach MM, Spann CO, Egan G. Effect of sleep deprivation on medical resident and student cognitive function: A prospective study. Am J Obstet Gynecol. 2003;188(5):1198-1201.
9. Van Dongen HP; Maislin G; Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003;26(3):117-126.
10. Goldin SB, Wahi MM, Farooq OS, et al. Student quality-of-life declines during third year surgical clerkship. J Surg Res. 2007;143(1):151-157.