Science has known for quite some time now that sleep is essential to forming long-term memories. In fact, a recent study by researchers at Brandeis University found that without sleep, memorization during all-night study sessions was completely inhibited. In a new study that sheds more light on the link between memory and sleep, researchers from Uppsala University in Sweden are finding that sleep is both essential to capturing long-term memories and accessing them under cognitive stress.

Publishing their findings in the journal SLEEP, Swedish researchers Jonathan Cedernaes and Christian Benedict explored the connection between the duration of sleep received each night, and the ability to recall long-term memories when the pressure is on. In order to do this, researchers studied 15 participants, giving them 15 card pairings on a computer screen and asking them to memorize as many as they can. Half of the subjects were then given four hours of sleep, and the other half was given a full night’s sleep of eight hours.

When asked the following day to recall the 15 card pairings from the previous day, researchers observed no difference on the impact of sleep duration on the subject’s ability to remember; those who got four hours of sleep were able to recall the pairings just as well as those who got eight hours.

However, when researchers exposed their subjects to cognitive stress, a significant difference in memorization was observed. After being exposed to either four or eight hours of sleep, researchers subjected participants to 30 minutes of acute stress the morning after. For example, while asking participants to recall a newly learned list of words, they were exposed to loud noise. Under this sort of stress, those who only received 4 hours of sleep recalled significantly less, showing an overall reduction in their ability to access long-term memory by about 10 percent. Those who received eight hours, though, did not show the same difficulty recalling long-term memories.

“On the basis of our study findings, we have two important take-home messages,” said Jonathan Cedernaes, of the Department of Neuroscience at Uppsala University, in a recent press release. “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.”

Throughout the school and work day we are presented with a variety of different stressors that impact our ability to process and recall information. When we lose sleep, this ability is further compromised, and we have an even more difficult time going through our day. Other studies have shown that the increased stress that often comes with lack of sleep can even lead to certain chronic conditions associated with memory, like Alzheimer’s.

Researchers believe that in the near future it will be necessary to study how lack of sleep and stress lead to these sorts of conditions, and what should be done to prevent them from happening. “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,” Cedernaes said.

Source: Cedernaes et al. Acute Sleep Deprivation Increases Serum Levels of Neuron-Specific Enolase (NSE) and S100 Calcium Binding Protein B (S-100B) in Healthy Young Men. SLEEP. 2015.

Haynes PR, Christmann BL, Griffith LC. A single pair of neurons links sleep to memory consolidation in Drosophila melanogaster. eLife. 2015.