Rocking at 1.0 Hz did, however, alter the spectral (EEG frequency) components of wakefulness and REM sleep, two states that exhibit predominant theta (~6-9 Hz) rhythmic oscillations. Specifically, 1.0 Hz rocking increased low-theta and decreased high-theta frequencies during both total wakefulness and a state known as ‘active’ or ‘theta-dominated wakefulness’ (TDW; see above). A shift in the spectral components of wakefulness from higher to lower frequencies is associated with building sleep pressure and a pending transition into sleep.

To investigate a mechanism for rhythmic rocking-induced sleep, the researchers tested whether otolithic organs of the vestibular system (which monitor our head’s linear acceleration) were necessary for the effect. They tested this using mice lacking functional otoliths (nicknamed tilted mice; Otop1-tlt/tlt). When they put these mice through their rocking experiment, they showed no enhancement of sleep like their counterparts with intact otolithic organs. A final question was whether the main driver of sleep was the rhythmic (i.e., frequency) component, or the linear acceleration applied to the mouse. To test this, they equalized the linear accelerations of both the 1.0 and 1.5 Hz rocking frequencies to 178 cm/s^2. When this was done, the effect on sleep was equalized, supporting the notion that linear acceleration, rather than frequency, is the important component of rocking-induced sleep. Interestingly, vestibular afferent nerves are 3-4 times less sensitive to stimuli than those in monkeys or humans. When the authors applied this conversion to the minimal sleep-enhancing linear acceleration that affected sleep in mice (79 cm/s^2), the numbers matched those that promote sleep in humans (20-26 cm/s^2).

An interesting note that the authors make is that other sensory modalities (e.g., vision, proprioception) could further be responsible for rocking-induced sleep, as tilted mice still had these systems intact. However, this is unlikely, as there was no compensatory effect in these mice, suggesting that the majority of the effect was driven by the vestibular system (see below).

A very interesting finding from this study is that the vestibular system contributes to sleep-wake control. As the authors discuss, future studies should examine downstream pathways relaying linear acceleration signals to known sleep circuitry (e.g., the pedunculopontine tegmentum) in the brain. So…next time you try to rock your baby or pet to sleep, remember that the linear acceleration is key!