Every night, while our bodies lie still, our brains churn through the day’s experiences. In the dark, memories are molded and secured through a dance of brainwaves — a rhythmic interplay between slow oscillations and faster bursts called spindles. Now, new findings suggest that these rhythms may be tuned, in part, by something as primal as hunger.
In a recent commentary in the European Journal of Neuroscience, neuroscientist Niels Niethard highlights intriguing evidence that fasting before sleep can sharpen the brain’s memory rhythms. The insights come in part from a new study in rats, where short-term fasting altered how brainwaves coordinated during sleep — fine-tuning a process that scientists believe plays a critical role in how memories are stored.
“Fasting led to a significant increase in the density of SOs [slow oscillations] and sleep spindles, as well as a higher rate of their co-occurrence,” Niethard writes.
Even more striking, hunger shifted the precise timing of these rhythms to a configuration “previously associated with enhanced memory consolidation during sleep.”
Fasting As a Hack for Learning?
For years, scientists have tracked how the brain replays memories during sleep. Slow oscillations — low-frequency waves that sweep through the cortex — alternate between silence and then brief bursts of activity. Sleep spindles, faster oscillations in the 11 to 16 Hz range, often emerge during these bursts. When the two patterns are tightly coupled — that is, when spindles align with the peaks of slow waves — the brain seems to be especially good at cementing new memories.
Depending on how this coupling occurs, it has been linked to learning, but also aging and cognitive decline. But whether the coordination is a hardwired trait or something shaped by daily experience has remained an open question.
Enter hunger.
In a 2025 study led by Yun Lun at the University of Tübingen and colleagues, researchers fasted adult rats for six hours before letting them sleep. During sleep, the animals’ brains not only produced more of these deep sleep rhythms, but the rhythms were also more tightly timed. Spindles landed later in the slow wave, a temporal shift previously shown to support memory retention.
Total sleep time and the balance of REM and non-REM stages remained unchanged. Instead, hunger appeared to tune the brain’s internal timing system for memory processing.
When rats were instead given a glucose injection that infused calories, some rhythms — like spindle density — increased. But others, including the alignment of slow waves and spindles, stayed the same. Only fasting altered the coupling in ways thought to enhance memory function.
Not Just Rats
Though the rodent data provide some of the clearest experimental evidence, human studies are beginning to point in the same direction.
A 2023 study by Raphael Vallat from the University of California, Berkeley found that in people, the precision of sleep coupling was tied to glucose regulation. Individuals with better fasting glucose levels the next morning had stronger and more precisely timed SO-spindle coupling the night before.
“These correlations remained significant after controlling for variables such as age, sex, race, BMI, hypertension, sleep apnea severity, and sleep duration,” Niethard notes. But when diabetes status was included, the link disappeared, suggesting that metabolic health may play a direct role in tuning sleep’s memory machinery.
The idea that sleep rhythms might be sensitive to what we eat — or don’t eat — has wide implications. While much research has focused on how sleep supports metabolism, these new findings flip the script. Metabolism may shape how well we sleep and remember things from our waking life.
A Flexible Memory System
Niethard’s commentary builds on a recent study by Nathan Cross et al. (2025), which examined whether learning tasks before bed could shift SO-spindle coupling in humans. The answer? Not significantly.
Although participants performed memory tasks before one night of sleep, and no such task on another, the coordination of slow waves and spindles did not change between nights. Some subtle associations did, however, emerge. In one group, people who learned word pairs to a performance standard showed a correlation between their memory performance and the phase of coupling. Still, the broad takeaway was that coupling appeared stable.
But while cramming learning before bed doesn’t seem to boost memory consolidation, fasting does, under the right conditions. “While factors like age set a general framework for SO-spindle coupling,” Niethard writes, “experience-dependent influences such as fasting can modulate its temporal dynamics.”
As scientists continue decoding how sleep builds memory, they are learning that the process may not just depend on what we learn — but when we eat. Hunger, it turns out, doesn’t just sharpen the senses. It might also help shape the brain’s internal rhythm, setting the stage for stronger memories by morning.
