Rocking and Sleeping

We are posting below an article containing interesting results on the relationship between rocking and sleep in a short nap.  Another reason to nap in the hammock:

Rocking
synchronizes brain
waves during a
short nap
Laurence Bayer1*,
Irina Constantinescu1*,
Stephen Perrig2, Julie Vienne3,
Pierre-Paul Vidal4, Michel Mühlethaler1*
and Sophie Schwartz1,5*
Why do we cradle babies or irresistibly
fall asleep in a hammock? Although
such simple behaviors are common
across cultures and generations, the
nature of the link between rocking
and sleep is poorly understood [1,2].
Here we aimed to demonstrate that
swinging can modulate physiological
parameters of human sleep. To
this end, we chose to study sleep
during an afternoon nap using
polysomnography and EEG spectral
analyses. We show that lying on a
slowly rocking bed (0.25 Hz) facilitates
the transition from waking to sleep,
and increases the duration of stage
N2 sleep. Rocking also induces a
sustained boosting of slow oscillations
and spindle activity. It is proposed
that sensory stimulation associated
with a swinging motion exerts a
synchronizing action in the brain that
reinforces endogenous sleep rhythms.
These results thus provide scientific
support to the traditional belief that
rocking can soothe our sleep.
In the present study, we asked
twelve healthy male volunteers (22–38
years old) to nap on a bed that could
either remain stationary or rock gently
(0.25 Hz; Figure 1A). All participants
were good sleepers, non-habitual
nappers with no excessive daytime
sleepiness and had low anxiety
levels. Sleep quality and quantity
were assessed by questionnaires
and actimetry recordings. The
experimental procedure involved
taking two 45-minute afternoon naps
(2:30 to 3:15 PM): one with the bed
stationary, and one with the bed put in
motion (condition order randomized).
The motion parameters were set to
stimulate vestibular and proprioceptive
sensory systems, without causing
nausea or any entrainment of cardiac
rhythm. In both conditions the naps
were spent in complete darkness in a
controlled room temperature (21 ± 1°C)
and the level of auditory stimulation
was around 37 dB. During both
sessions, polysomnography data were
recorded continuously. Sleep stages
and sleep spindles were visually
identified by two experienced scorers,
blind to the experimental conditions.
We also performed spectral analysis
(FFT routine) using the midline frontal
(Fz) and parietal (Pz) derivations.
The data from two participants were
excluded from the final analyses (see
the Supplemental Information).
Over the three consecutive nights
preceding each experimental day, all
participants had a good quality and
quantity (mean ± s.e.m.; 7.32 ± 0.78 h)
of sleep as assessed by self-rated
sleep questionnaires, with no difference
for these measurements between
stationary and swinging conditions.
Similarly, wrist actimetry recorded
during these same nights did not
show any difference in sleep efficiency
between conditions (mean ± s.e.m.;
swinging: 86.63 ± 1.95%; stationary:
86.71 ± 1.23%). For both conditions,
participants were more alert (on
visual analogue scale) after napping
than before (F(1,9) = 8.4, P = 0.018).
Eight participants rated the swinging
condition as ‘more pleasant’ than the
stationary condition; for one participant
both sessions were equally pleasant
and for one participant the stationary
condition was more pleasant.
We found that rocking accelerated
sleep onset, as evidenced by a
shorter duration of stage N1 sleep
and a reduction of stage N2 latency,
compared to the stationary condition
(Supplemental Table S1). Rocking
also affected deeper sleep stages by
increasing the duration of stage N2
sleep and the mean spindle density per
30-s epoch (Supplemental Table S1,
Figure 1B). Spindle density increased
significantly from the second half of
the nap (Figure 1C) and persisted
throughout the entire duration of stage
N2 (Supplemental Figure S1A). All
these modifications were observed in
each and every participant (all
P < 0.009; Supplemental Table
S1). In the only previous study
investigating the effect of rocking
on sleep, Woodward et al. [1] found
no consistent modulation for the
percentage of stage 1 sleep and an
overall reduction of the percentage
of stage 2 sleep during the motion
condition. In contrast to our present
study, however, these data were
computed over whole nights of sleep
recordings, and did not address
the question of whether vestibular/
somatosensory inputs influence the
transition from wakefulness to sleep
(stage 1 and 2 sleep early in the night
after sleep onset).
Rocking also increased EEG power
of slow wave activity (SWA: 0.6–5 Hz;
Figure 1D), predominantly during the
last third of stage N2 (Supplemental
Figure S1B; P < 0 .005). A significant
increase of EEG power within spindle
frequency bands was also observed
for the frontal derivation (P < 0.05;
Figure1D and Supplemental Figure
S1C), but not for the parietal derivation
(P > 0.07; Supplemental Results) [3].
Together these results show that
rocking induces a speeded transition
to an unambiguous sleep state, and
may enhance sleep by boosting slow
oscillations and spindle activity.
How can we explain that rocking
may accelerate wake-sleep transition
and promote sleep consolidation?
Three mechanisms could explain
these effects of rocking on sleep. First,
because vestibular/somatosensory
pathways have anatomical links with
structures implicated in emotions
such as the amygdala [4] and
because the amygdala affects the
regulation of sleep-wake states [5],
faster sleep onset could be due to
a ‘relaxing’ feeling associated with
the rocking condition, which most of
our participants (8 out of 10) found
pleasant. Second, rhythmic vestibular/
somatosensory inputs associated with
rocking may modulate sleep-wake
centres via direct or indirect
connections between sensory systems
and hypothalamic [6] or brainstem
areas [7]. Third, sensory inputs could
affect the synchrony of neural activity
within thalamo-cortical networks
because both somatosensory
and vestibular inputs send direct
projections to thalamic nuclei [8].
Consistent with this view, slow rhythmic
cortical stimulation was recently found
to increase EEG slow oscillations and
spindles [3,9], which are both hallmarks
of deep sleep. The latter hypothesis
of an influence on neural synchrony
fits best the present observation that
rocking does not only facilitate sleep
onset but has a persistent effect on
brain oscillations and spindles. Recent
evidence that increased spindle activity
protects sleep against disruptive stimuli
is in agreement with this interpretation
[10]. Follow-up experiments could
assess whether sleep changes
triggered by rocking have beneficial
functional consequences on
post-sleep performance or on memory
consolidation processes [3].
We suggest that rhythmic rocking
may enhance synchronous activity
within thalamo-cortical networks,
which in turn could promote the onset
of sleep and its maintenance. The use
of rocking to soothe sleep thus belongs
to our repertoire of adaptive behaviours
in which a natural mechanism of sleep
(thalamo-cortical synchronization) has
been harnessed in the simplest manner
since immemorial times.
Supplemental Information
Supplemental Information includes one
figure, one table, Supplemental Results and
Supplemental Experimental Procedures,
and can be found with this article online at
doi:10.1016/j.cub.2011.05.012.
Acknowledgments
This work was supported by the Swiss
National Science Foundation. We thank
A. Borbély, P. Franken, C. Frith, B.E. Jones,
C. Leonard, and M. Tafti for their comments.
References
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1Department of Neuroscience, University
of Geneva, Switzerland. 2Sleep Laboratory,
Geneva University Hospital, Switzerland.
3CIG, University of Lausanne, Switzerland.
4CNRS, UMR 8194-Université Paris
Descartes, France. 5Swiss Center for
Affective Sciences, University of Geneva,
Switzerland.
*These authors contributed equally to the
work.
E-mail: sophie.schwartz@unige.ch;
michel.muhlethaler@unige.ch
Correspondencetable

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