Memories lost to sleep deprivation aren't gone — they're just locked away, study finds

Most people have experienced the unsettling moment after a sleepless night when an important name, face, or appointment simply vanishes from memory.

A new study suggests that what feels like forgetting may not be forgetting at all. The memories are not erased — they are hidden somewhere in the brain, inaccessible but intact. Animal experiments have shown that both light stimulation and drug treatment can unlock them.

The findings, published in the June 2026 issue of Science Advances, come from a research team led by professor Robbert Havekes at the University of Groningen in the Netherlands. The team examined how sleep deprivation affects long-term social recognition memory and identified the neuroscientific mechanisms that could reverse the damage.

Sleep loss makes the brain forget social bonds

The research team focused on social memory — the ability to recognize specific individuals and maintain relationships with them. This function is essential to social life, and its breakdown is a hallmark of conditions such as Alzheimer's disease.

In the experiment, mice were allowed to interact with an unfamiliar mouse for 10 minutes. Half were then kept awake for six hours. When all the mice were reintroduced to the same individual 24 hours later, the two groups behaved very differently.

The effect of sleep deprivation on social memory in mice. (G·H, first encounter) During the initial interaction, both well-rested mice (NSD) and sleep-deprived mice (SD) showed strong curiosity and preference toward the unfamiliar mouse (green bars) over a familiar one. (I·J, second encounter) When reintroduced 24 hours later, the well-rested mice (NSD) recognized the individual and showed reduced interest, while the sleep-deprived mice (SD) showed no recognition — sniffing and exploring the familiar mouse at length (green bars) as if meeting it for the first time. [Science Advances, June 2026]

The well-rested mice recognized the mouse they had met the previous day and spent less time investigating it. The sleep-deprived mice explored it just as intently as before, as though encountering it for the first time. On the surface, the memory appeared to be gone.

To test whether the memory had truly been erased, the research team ran two separate experiments aimed at recovering it.

Switching memory cells back on with light

The clusters of neurons that store a specific experience are known as engrams, or memory cells. The concept holds that cells activated together during an experience become jointly responsible for encoding that memory.

The research team tagged the memory cells active during the original social encounter with a fluorescent marker, then used optogenetics — a technique that uses light to stimulate specific neurons — to reactivate those cells later.

When the memory cells of sleep-deprived mice were reactivated with light, the mice recognized the individual they had met the day before. The memory had not been erased; it had been stored in the brain all along, simply beyond reach.

The results grew more striking still. A single pulse of light stimulation was enough: when the mice were tested six days later, they still recognized the individual.

The researchers offered this analogy: the contents of a drawer were always there, but the key had been lost. The light stimulation forced the drawer open once, and after that the mice could open it on their own.

Microscopic images and verification data for memory cells reactivated by light. (F, brain cell microscopy) An image of the hippocampus showing "memory cells" (red) formed during a social encounter and "active cells" (green) firing during memory recall — the two populations overlap precisely, glowing yellow (arrows inside circles). (G·H·I, memory retention) Even in sleep-deprived mice with impaired recall, the total number of memory cells in the brain (G, H) was preserved at the same level as in well-rested mice. After light stimulation, the memory cells aligned accurately and the proportion of overlapping yellow cells (I) jumped sharply. [Science Advances, June 2026]

An existing drug unlocks the same blocked memories

The research team also tested a pharmacological approach that could be applied in clinical settings.

The drug in question was roflumilast, already approved by the US Food and Drug Administration for treatment of chronic obstructive pulmonary disease. It works by preventing the breakdown of a signaling molecule inside cells that is required for memory formation.

Administering the drug to sleep-deprived mice prevented memory impairment from developing. Even in mice that had already lost the memory, a dose given 30 minutes before a recall test restored it temporarily.

The research team interpreted the result as the drug reopening the neural pathway that sleep deprivation had blocked.

Distinct memories are stored separately

The study also revealed that when mice encountered two different individuals on consecutive days in the same space, the brain stored each memory separately.

The research team labeled the memory cells activated during each encounter with different colored fluorescent markers. The two sets of cells overlapped by only about 30 percent, and that proportion held regardless of whether the mice had been sleep-deprived.

When only the memory cells from the first encounter were stimulated with light, the mice recognized only the first individual. Selective retrieval of a specific memory was possible. Sleep deprivation did not affect the initial formation of memory cells.

New clues for treating sleep disorders and dementia

Alzheimer's disease. [Getty Images Bank]

The pattern of memories appearing lost while in fact remaining inaccessible has been reported in animal models of early-stage Alzheimer's disease as well, suggesting the phenomenon may extend beyond sleep deprivation.

Because roflumilast is already FDA-approved for use in humans, the findings open the door to potential applications in treating memory decline caused by sleep disorders or aging.

"This could become a new medical clue for treating memory decline caused by sleep disorders, aging, and related conditions," the research team said.

However, the team acknowledged a limitation: the experiments were conducted exclusively on male mice, and further validation will be needed before the findings can be applied to humans.

Reference

DOI: 10.1126/sciadv.adu9805

Adithya Sarma et al., "Restoring access to long-term social recognition memories disrupted by sleep deprivation." Sci. Adv. 12, eadu9805 (2026).