The Neuroscience of ‘Rock-a-Bye Baby’

I've always wondered why I sleep like a baby when on a boat being slowly rocked by waves, so was intrigued by Friedman's recent piece pointing to work by Perrault et al. showing that a slow rocking motion not only improves sleep but also can help people consolidate memories overnight. This is because continuous rocking stimulation strengthens deep sleep via the neural entrainment of intrinsic sleep oscillations. The Perrault et al. summary:

Highlights

•Rocking boosts deep sleep, sleep maintenance, and memory in healthy sleepers

•Fast spindles increase during rocking and synchronize with the slow oscillation up-state

• Rocking-induced overnight memory improvement relates to increased sigma activity

• Continuous rocking stimulation actively entrains intrinsic sleep oscillations

Summary

Sensory processing continues during sleep and can influence brain oscillations. We previously showed that a gentle rocking stimulation (0.25 Hz), during an afternoon nap, facilitates wake-sleep transition and boosts endogenous brain oscillations (i.e., EEG spindles and slow oscillations [SOs]). Here, we tested the hypothesis that the rhythmic rocking stimulation synchronizes sleep oscillations, a neurophysiological mechanism referred to as “neural entrainment.” We analyzed EEG brain responses related to the stimulation recorded from 18 participants while they had a full night of sleep on a rocking bed. Moreover, because sleep oscillations are considered of critical relevance for memory processes, we also investigated whether rocking influences overnight declarative memory consolidation. We first show that, compared to a stationary night, continuous rocking shortened the latency to non-REM (NREM) sleep and strengthened sleep maintenance, as indexed by increased NREM stage 3 (N3) duration and fewer arousals. These beneficial effects were paralleled by an increase in SOs and in slow and fast spindles during N3, without affecting the physiological SO-spindle phase coupling. We then confirm that, during the rocking night, overnight memory consolidation was enhanced and also correlated with the increase in fast spindles, whose co-occurrence with the SO up-state is considered to foster cortical synaptic plasticity. Finally, supporting the hypothesis that a rhythmic stimulation entrains sleep oscillations, we report a temporal clustering of spindles and SOs relative to the rocking cycle. Altogether, these findings demonstrate that a continuous rocking stimulation strengthens deep sleep via the neural entrainment of intrinsic sleep oscillations.