The brain has “two-track” approach to sleep, maintaining a basic alert system through the lower brain, while deliberately reducing higher-level processing to prioritize rest and memory consolidation.
When we drift off to sleep, our brains face a delicate challenge: they must protect our rest and consolidate memories while remaining alert to potentially important sounds, like an alarm or a baby crying. New research led by Hugo R. Jourde, who conducted the work as a doctoral student in Concordia’s Department of Psychology and has since graduated, sheds light on how the brain manages this balancing act, offering fresh insight into the complex relationship between sleep, memory, and our responsiveness to the outside world.
The study, conducted with 14 adult participants, focused on a brain signal known as the frequency-following response (FFR). The FFR is measured using a technique known as electroencephalography. It reveals how well the brain can track the pitch of sounds, and it originates from multiple levels of the auditory system – from the deep brainstem to higher-level areas like the auditory cortex. By recording participants’ brain activity during daytime naps, the team was able to track how these different regions processed sounds across varying stages of sleep.
The results indicate that during lighter and intermediate stages of sleep, the lower brain regions – particularly the brainstem – remained active, reliably following the sounds presented. But as participants entered deep sleep, the higher cortical regions became less responsive. Importantly, this shift wasn’t linked to specific sleep events, such as slow waves or spindles. Instead, the data pointed to a reduction in communication between the thalamus, a relay hub in the middle of the brain, and the auditory cortex.
According to the researchers, this pattern reveals the brain’s “two-track” approach to sleep. On one hand, it maintains a basic alert system through the lower brain, ensuring we can wake up when necessary. On the other, it may deliberately reduce higher-level processing to prioritize rest and memory consolidation.
“This work helps us understand why sleep is both protective and restorative,” explains Emily B. J. Coffey, professor in the Department of Psychology. “The brain doesn’t completely shut off; it listens selectively, keeping us safe without sacrificing recovery.”
The study also provides a foundation for understanding why these systems may weaken with age. Older adults often experience lighter, more easily disrupted sleep. The researchers suggest that changes in communication between the thalamus and cortex may play a role, potentially contributing to more fragmented rest.
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