It is well known that during sleep newly learned information is transferred from short-term to long-term memory stores in humans. In a new study, sleep researchers Jonathan Cedernaes and Christian Benedict, sought to investigate the role of nocturnal sleep duration for this memory transfer, and how long-term memories formed by sleep remain accessible after acute cognitive stress.
Following a learning session in the evening during which 15 participants learned 15 card pair locations on a computer screen, in one experimental session subjects slept for half a night (4-hr) and in the other for a full night (8-hr). The next morning subjects were asked to recall as many card pair locations as possible. What the researchers found was that half a night of sleep (4-hr) was as powerful as a full night of sleep (8-hr) to form long-term memories for the learned card pair locations.
However, the study also revealed that stress had an impact on the participants’ ability to recall these memories. The men were acutely stressed for 30 minutes in the morning after a half or full night of sleep (for example by having to recall a newly learnt list of words while exposed to noise). Following short sleep this stress exposure reduced their ability to recall these card pair locations by around 10%. In contrast, no such stress-induced impairment was seen when the same men were allowed to sleep for a full night.
“On the basis of our study findings, we have two important take home messages: First, even though losing half a night of sleep may not impair memory functions under baseline conditions, the addition of acute cognitive stress may be enough to lead to significant impairments, which can possibly be detrimental in real-world scenarios. Second, interventions such as delaying school start times and greater use of flexible work schedules, that increase available snooze time for those who are on habitual short sleep, may improve their academic and occupational performance by ensuring optimal access to memories under stressful conditions”, says Cedernaes, researcher at the department of neuroscience, Uppsala University.
“An important next step will be to investigate how chronic sleep loss and or more chronic stress may interact to impair the ability to retrieve memories that are consolidated during sleep”, says Cedernaes.
STUDY OBJECTIVE: Our study sought to investigate the role of nocturnal sleep duration for the retrieval of oversleep consolidated memories, both prior to and after being cognitively stressed for ~30 minutes the next morning. DESIGN: Participants learned object locations (declarative memory task comprising 15 card pairs) and a finger tapping sequence (procedural memory task comprising 5 digits) in the evening. After learning, participants either had a sleep opportunity of 8 hours (between ~23:00 and ~07:00, full sleep condition) or they could sleep between ~03:00 and ~07:00 (short sleep condition). Retrieval of both memory tasks was tested in the morning after each sleep condition, both before (~08:30) and after being stressed (~09:50). SETTING: Sleep laboratory. PARTICIPANTS: 15 healthy young men. RESULTS: Our analysis demonstrated that oversleep memory changes did not differ between sleep conditions. However, in their short sleep condition, following stress hallmarked by increased subjective stress feelings, the men were unable to maintain their pre-stress performance on the declarative memory task, whereas their performance on the procedural memory task remained unchanged. While men felt comparably subjectively stressed by the stress intervention, overall no differences between pre- and post-stress recalls were observed following a full night of sleep. CONCLUSIONS: Our findings suggest that 8-h sleep duration, within the range recommended by the US National Sleep Foundation, may not only help consolidate newly learned procedural and declarative memories, but also ensure full access to both during periods of subjective stress.